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[[cha:code-notes]]
= Code Notes

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== Intended audience

This document is a collection of notes about the internals of LinuxCNC. It
is primarily of interest to developers, however much of the information
here may also be of interest to system integrators and others who are
simply curious about how LinuxCNC works. Much of this information is now
outdated and has never been reviewed for accuracy.

== Organization

There will be a chapter for each of the major components of LinuxCNC, as
well as chapter(s) covering how they work together. This document is
very much a work in progress, and its layout may change in the future.

== Terms and definitions

* 'AXIS' - An axis is one of the nine degrees of freedom that define a tool
  position in three-dimensional Cartesian space. Those nine axes are
  referred to as X, Y, Z, A, B, C, U, V, and W. The linear orthogonal
  coordinates X, Y, and Z determine where the tip of the tool is
  positioned. The angular coordinates A, B, and C determine the tool
  orientation. A second set of linear orthogonal coordinates U, V, and W
  allows tool motion (typically for cutting actions) relative to the
  previously offset and rotated axes.
  Unfortunately "axis" is also
  sometimes used to mean a degree of freedom of the machine itself, such
  as the saddle, table, or quill of a Bridgeport type milling machine. On
  a Bridgeport this causes no confusion, since movement of the table
  directly corresponds to movement along the X axis. However, the
  shoulder and elbow joints of a robot arm and the linear actuators of a
  hexapod do not correspond to movement along any Cartesian axis, and in
  general it is important to make the distinction between the Cartesian
  axes and the machine degrees of freedom. In this document, the latter
  will be called 'joints', not axes. The GUIs and some other parts of
  the code may not always follow this distinction, but the internals of
  the motion controller do.

* 'JOINT' - A joint is one of the movable parts of the machine. Joints are
  distinct from axes, although the two terms are sometimes (mis)used to
  mean the same thing. In LinuxCNC, a joint is a physical thing that can be
  moved, not a coordinate in space. For example, the quill, knee, saddle,
  and table of a Bridgeport mill are all joints. The shoulder, elbow, and
  wrist of a robot arm are joints, as are the linear actuators of a
  hexapod. Every joint has a motor or actuator of some type associated
  with it. Joints do not necessarily correspond to the X, Y, and Z axes,
  although for machines with trivial kinematics that may be the case.
  Even on those machines, joint position and axis position are
  fundamentally different things. In this document, the terms 'joint' and
  'axis' are used carefully to respect their distinct meanings.
  Unfortunately that isn't necessarily true everywhere else. In
  particular, GUIs for machines with trivial kinematics may gloss over or
  completely hide the distinction between joints and axes. In addition,
  the INI file uses the term 'axis' for data that would more accurately
  be described as joint data, such as input and output scaling, etc.

[NOTE]
This distinction was made in version 2.8 of LinuxCNC.
The INI file got a new section [JOINT_<num>]. Many of the parameters that were previously
proper to the [AXIS_<letter>] section are now in the new section. Other sections,
such as [KINS], also take on new parameters to match this.
An update script has been provided to transform old INI files to the new axes/joints configuration.

* 'POSE' - A pose is a fully specified position in 3D Cartesian space. In
  the LinuxCNC motion controller, when we refer to a pose we mean an
  EmcPose structure, containing six linear coordinates (X, Y, Z, U,
  V, and W) and three angular ones (A, B, and C).

* 'coord', or coordinated mode, means that all articulations are synchronized and
  they move together as directed by the higher-level code. It is the normal mode when machining.
  In coordinated mode, commands are assumed to be given in the Cartesian reference frame,
  and if the machine is not Cartesian, the commands are translated by the kinematics to drive
  each joint into the joint space as needed.

* 'free' means that commands are interpreted in joint space.
  It is used to manually move (jog) individual joints, although it does not prevent them from moving
  multiple joints at once (I think).
  Homing is also done in free mode; in fact, machines with non-trivial kinematics
  must be homed before they can go into coord or teleop mode.

* 'teleop' is the mode you probably need if you are jogging with a hexapod.
  The jog commands implemented by the motion controller are joint jogs, which
  work in free mode. But if you want to move a hexapod or similar machine along a
  cartesian axis in particular, you must operate more than one joint.
  That's what 'teleop' is for.

== Architecture overview

There are four components contained in the LinuxCNC Architecture: a motion
controller (EMCMOT), a discrete IO controller (EMCIO), a task executor
which coordinates them (EMCTASK) and several text-mode and graphical
User Interfaces. Each of them will be described in the current
document, both from the design point of view and from the developers
point of view (where to find needed data, how to easily extend/modify
things, etc.).

image::LinuxCNC-block-diagram-small.png[align="center"]

=== LinuxCNC software architecture

At the coarsest level, LinuxCNC is a
hierarchy of three controllers: the task level command handler and program
interpreter, the motion controller, and the discrete I/O controller. The
discrete I/O controller is implemented as a hierarchy of controllers,
in this case for spindle, coolant, and auxiliary (e.g., estop, lube)
subsystems. The task controller coordinates the actions of the motion and
discrete I/O controllers. Their actions are programmed in conventional
numerical control "G and M code" programs, which are interpreted by
the task controller into NML messages and sent to either the motion or
discrete I/O controllers at the appropriate times.

== Motion Controller Introduction

The motion controller receives commands from user space modules via a
shared memory buffer, and executes those commands in realtime. The
status of the controller is made available to the user space modules
through the same shared memory area. The motion controller interacts
with the motors and other hardware using the HAL (Hardware Abstraction
Layer). This document assumes that the reader has a basic understanding
of the HAL, and uses terms like HAL pins, HAL signals, etc, without
explaining them. For more information about the HAL, see the
HAL Manual. Another chapter of this document will
eventually go into the internals of the HAL itself, but in this
chapter, we only use the HAL API as defined in src/hal/hal.h.

=== Motion Controller Modules

The realtime functions of the motion controller are implemented
with modules -- shared objects for rtpreempt systems or kernel
modules for some implementations (RTAI):

* 'tpmod' - trajectory planning
* 'homemod' - homing functions
* 'motmod' - processes NML commands and controls hardware via hal
* 'kinematics module' - performs forward (joints-->coordinates) and
  inverse (coordinates->joints) kinematics calculations

LinuxCNC is started by a *linuxcnc* script which reads a
configuration INI file and starts all needed processes.  For
realtime motion control, the script first loads the default tpmod
and homemod modules and then loads the kinematics and motion
modules according to settings in halfiles specified by the INI
file.

Custom (user-built) homing or trajectory-planning modules can
be used in place of the default modules via INI file settings
or command line options.  Custom modules must implement all
functions used by the default modules.  The halcompile utility
can be used to create a custom module.

image::LinuxCNC-motion-controller-small.png[align="center"]

== Block diagrams and Data Flow

The following figure is a block diagram
of a joint controller. There is one joint controller per joint. The
joint controllers work at a lower level than the kinematics, a level
where all joints are completely independent. All the data for a joint
is in a single joint structure. Some members of that structure are
visible in the block diagram, such as coarse_pos, pos_cmd, and
motor_pos_fb.

.Joint Controller Block Diagram
image::emc2-motion-joint-controller-block-diag.png[align="center"]

The above figure shows five of the
seven sets of position information that form the main data flow through
the motion controller. The seven forms of position data are as follows:

* 'emcmotStatus\->carte_pos_cmd' - This is the desired position, in
  Cartesian coordinates. It is updated at the traj rate, not the servo
  rate. In coord mode, it is determined by the traj planner. In teleop
  mode, it is determined by the traj planner? In free mode, it is either
  copied from actualPos, or generated by applying forward kins to (2) or
  (3).
* 'emcmotStatus\->joints[n].coarse_pos' - This is the desired position, in
  joint coordinates, but before interpolation. It is updated at the traj
  rate, not the servo rate. In coord mode, it is generated by applying
  inverse kins to (1) In teleop mode, it is generated by applying inverse
  kins to (1) In free mode, it is copied from (3), I think.
* 'emcmotStatus\->joints[n].pos_cmd - This is the desired position, in
  joint coords, after interpolation. A new set of these coords is
  generated every servo period. In coord mode, it is generated from (2)
  by the interpolator. In teleop mode, it is generated from (2) by the
  interpolator. In free mode, it is generated by the free mode traj
  planner.
* 'emcmotStatus\->joints[n].motor_pos_cmd' - This is the desired position,
  in motor coords. Motor coords are generated by adding backlash
  compensation, lead screw error compensation, and offset (for homing) to
  (3). It is generated the same way regardless of the mode, and is the
  output to the PID loop or other position loop.
* 'emcmotStatus\->joints[n].motor_pos_fb' - This is the actual position, in
  motor coords. It is the input from encoders or other feedback device
  (or from virtual encoders on open loop machines). It is "generated" by
  reading the feedback device.
* 'emcmotStatus\->joints[n].pos_fb' - This is the actual position, in joint
  coordinates. It is generated by subtracting offset, lead screw error
  compensation, and backlash compensation from (5). It is generated the
  same way regardless of the operating mode.
* 'emcmotStatus\->carte_pos_fb' - This is the actual position, in Cartesian
  coordinates. It is updated at the traj rate, not the servo rate.
  Ideally, actualPos would always be calculated by applying forward
  kinematics to (6). However, forward kinematics may not be available, or
  they may be unusable because one or more axes aren't homed. In that
  case, the options are: A) fake it by copying (1), or B) admit that we
  don't really know the Cartesian coordinates, and simply don't update
  actualPos. Whatever approach is used, I can see no reason not to do it
  the same way regardless of the operating mode. I would propose the
  following: If there are forward kins, use them, unless they don't work
  because of unhomed axes or other problems, in which case do (B). If no
  forward kins, do (A), since otherwise actualPos would _never_ get
  updated.

== Homing

=== Homing state diagram

image::homing.svg[align="center"]

=== Another homing diagram

image::hss.svg[align="center"]

== Commands

This section simply lists all of the commands that can be sent to the
motion module, along with detailed explanations of what they do. The
command names are defined in a large typedef enum in
{linuxcnc}/src/emc/motion/motion.h, called cmd_code_t. (Note that in the
code, each command name starts with 'EMCMOT_', which is omitted here.)

The commands are implemented by a large switch statement in the
function emcmotCommandHandler(), which is called at the servo rate.
More on that function later.

There are approximately 44 commands - this list is still under
construction.

[NOTE]
The cmd_code_t enumeration, in motion.h, contains 73 commands, but the switch
statement in command.c contemplates only 70 commands (as of 6/5/2020).
ENABLE_WATCHDOG / DISABLE_WATCHDOG commands are in motion-logger.c. Maybe they are obsolete.
The SET_TELEOP_VECTOR command only appears in motion-logger.c, with no effect other than its own log.

=== ABORT

The ABORT command simply stops all motion. It can be issued at any
time, and will always be accepted. It does not disable the motion
controller or change any state information, it simply cancels any
motion that is currently in progress.footnote:[It seems that the
higher level code (TASK and above) also use ABORT to clear faults.
Whenever there is a persistent fault (such as being outside the
hardware limit switches), the higher level code sends a constant
stream of ABORTs to the motion controller trying to make the
fault go away. Thousands of them.... That means that the motion
controller should avoid persistent faults. This needs to be looked
into.]

==== Requirements

None. The command is always accepted and acted on immediately.

==== Results

In free mode, the free mode trajectory planners are disabled. That
results in each joint stopping as fast as its accel (decel) limit
allows. The stop is not coordinated. In teleop mode, the commanded
Cartesian velocity is set to zero. I don't know exactly what kind of
stop results (coordinated, uncoordinated, etc), but will figure it out
eventually. In coord mode, the coord mode trajectory planner is told to
abort the current move. Again, I don't know the exact result of this,
but will document it when I figure it out.

=== FREE

The FREE command puts the motion controller in free mode. Free mode
means that each joint is independent of all the other joints. Cartesian
coordinates, poses, and kinematics are ignored when in free mode. In
essence, each joint has its own simple trajectory planner, and each
joint completely ignores the other joints. Some commands (like Joint
JOG and HOME) only work in free mode. Other commands, including anything
that deals with Cartesian coordinates, do not work at all in free mode.

==== Requirements

The command handler applies no requirements to the FREE command, it
will always be accepted. However, if any joint is in motion
(GET_MOTION_INPOS_FLAG() == FALSE), then the command will be ignored.
This behavior is controlled by code that is now located in the function
'set_operating_mode()' in control.c, that code needs to be cleaned up.
I believe the command should not be silently ignored, instead the
command handler should determine whether it can be executed and return
an error if it cannot.

==== Results

If the machine is already in free mode, nothing. Otherwise, the
machine is placed in free mode. Each joint's free mode trajectory
planner is initialized to the current location of the joint, but the
planners are not enabled and the joints are stationary.

=== TELEOP

The TELEOP command places the machine in teleoperating mode. In teleop
mode, movement of the machine is based on Cartesian coordinates using
kinematics, rather than on individual joints as in free mode. However
the trajectory planner per se is not used, instead movement is
controlled by a velocity vector. Movement in teleop mode is much like
jogging, except that it is done in Cartesian space instead of joint
space. On a machine with trivial kinematics, there is little difference
between teleop mode and free mode, and GUIs for those machines might
never even issue this command. However for non-trivial machines like
robots and hexapods, teleop mode is used for most user commanded jog
type movements.

==== Requirements

The command handler will reject the TELEOP command with an error
message if the kinematics cannot be activated because the one or more
joints have not been homed. In addition, if any joint is in motion
(GET_MOTION_INPOS_FLAG() == FALSE), then the command will be ignored
(with no error message). This behavior is controlled by code that is
now located in the function 'set_operating_mode()' in control.c. I
believe the command should not be silently ignored, instead the command
handler should determine whether it can be executed and return an error
if it cannot.

==== Results

If the machine is already in teleop mode, nothing. Otherwise the
machine is placed in teleop mode. The kinematics code is activated,
interpolators are drained and flushed, and the Cartesian velocity
commands are set to zero.

=== COORD

The COORD command places the machine in coordinated mode. In coord
mode, movement of the machine is based on Cartesian coordinates using
kinematics, rather than on individual joints as in free mode. In
addition, the main trajectory planner is used to generate motion, based
on queued LINE, CIRCLE, and/or PROBE commands. Coord mode is the mode
that is used when executing a G-code program.

==== Requirements

The command handler will reject the COORD command with an error
message if the kinematics cannot be activated because the one or more
joints have not been homed. In addition, if any joint is in motion
(GET_MOTION_INPOS_FLAG() == FALSE), then the command will be ignored
(with no error message). This behavior is controlled by code that is
now located in the function 'set_operating_mode()' in control.c. I
believe the command should not be silently ignored, instead the command
handler should determine whether it can be executed and return an error
if it cannot.

==== Results

If the machine is already in coord mode, nothing. Otherwise, the
machine is placed in coord mode. The kinematics code is activated,
interpolators are drained and flushed, and the trajectory planner
queues are empty. The trajectory planner is active and awaiting a LINE,
CIRCLE, or PROBE command.

=== ENABLE

The ENABLE command enables the motion controller.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

If the controller is already enabled, nothing. If not, the controller
is enabled. Queues and interpolators are flushed. Any movement or
homing operations are terminated. The amp-enable outputs associated
with active joints are turned on. If forward kinematics are not
available, the machine is switched to free mode.

=== DISABLE

The DISABLE command disables the motion controller.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

If the controller is already disabled, nothing. If not, the controller
is disabled. Queues and interpolators are flushed. Any movement or
homing operations are terminated. The amp-enable outputs associated
with active joints are turned off. If forward kinematics are not
available, the machine is switched to free mode.

=== ENABLE_AMPLIFIER

The ENABLE_AMPLIFIER command turns on the amp enable output for a
single output amplifier, without changing anything else. Can be used to
enable a spindle speed controller.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

Currently, nothing. (A call to the old extAmpEnable function is
currently commented out.) Eventually it will set the amp enable HAL pin
true.

=== DISABLE_AMPLIFIER

The DISABLE_AMPLIFIER command turns off the amp enable output for a
single amplifier, without changing anything else. Again, useful for
spindle speed controllers.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

Currently, nothing. (A call to the old extAmpEnable function is
currently commented out.) Eventually it will set the amp enable HAL pin
false.

=== ACTIVATE_JOINT

The ACTIVATE_JOINT command turns on all the calculations associated
with a single joint, but does not change the joint's amp enable output
pin.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

Calculations for the specified joint are enabled. The amp enable pin
is not changed, however, any subsequent ENABLE or DISABLE commands will
modify the joint's amp enable pin.

=== DEACTIVATE_JOINT

The DEACTIVATE_JOINT command turns off all the calculations associated
with a single joint, but does not change the joint's amp enable output
pin.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

Calculations for the specified joint are enabled. The amp enable pin
is not changed, and subsequent ENABLE or DISABLE commands will not
modify the joint's amp enable pin.

=== ENABLE_WATCHDOG

The ENABLE_WATCHDOG command enables a hardware based watchdog (if
present).

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

Currently nothing. The old watchdog was a strange thing that used a
specific sound card. A new watchdog interface may be designed in the
future.

=== DISABLE_WATCHDOG

The DISABLE_WATCHDOG command disables a hardware based watchdog (if
present).

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

Currently nothing. The old watchdog was a strange thing that used a
specific sound card. A new watchdog interface may be designed in the
future.

=== PAUSE

The PAUSE command stops the trajectory planner. It has no effect in
free or teleop mode. At this point I don't know if it pauses all motion
immediately, or if it completes the current move and then pauses before
pulling another move from the queue.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

The trajectory planner pauses.

=== RESUME

The RESUME command restarts the trajectory planner if it is paused. It
has no effect in free or teleop mode, or if the planner is not paused.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

The trajectory planner resumes.

=== STEP

The STEP command restarts the trajectory planner if it is paused, and
tells the planner to stop again when it reaches a specific point. It
has no effect in free or teleop mode. At this point I don't know
exactly how this works. I'll add more documentation here when I dig
deeper into the trajectory planner.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

The trajectory planner resumes, and later pauses when it reaches a
specific point.

=== SCALE

The SCALE command scales all velocity limits and commands by a
specified amount. It is used to implement feed rate override and other
similar functions. The scaling works in free, teleop, and coord modes,
and affects everything, including homing velocities, etc. However,
individual joint velocity limits are unaffected.

==== Requirements

None. The command can be issued at any time, and will always be
accepted.

==== Results

All velocity commands are scaled by the specified constant.

=== OVERRIDE_LIMITS

The OVERRIDE_LIMITS command prevents limits from tripping until the
end of the next JOG command. It is normally used to allow a machine to
be jogged off of a limit switch after tripping. (The command can
actually be used to override limits, or to cancel a previous override.)

==== Requirements

None. The command can be issued at any time, and will always be
accepted. (I think it should only work in free mode.)

==== Results

Limits on all joints are over-ridden until the end of the next JOG
command. (This is currently broken... once an OVERRIDE_LIMITS command
is received, limits are ignored until another OVERRIDE_LIMITS command
re-enables them.)

=== HOME

The HOME command initiates a homing sequence on a specified joint. The
actual homing sequence is determined by a number of configuration
parameters, and can range from simply setting the current position to
zero, to a multi-stage search for a home switch and index pulse,
followed by a move to an arbitrary home location. For more information
about the homing sequence, see the homing section of the Integrator Manual.

==== Requirements

The command will be ignored silently unless the machine is in free mode.

==== Results

Any jog or other joint motion is aborted, and the homing sequence
starts.

=== JOG_CONT

The JOG_CONT command initiates a continuous jog on a single joint. A
continuous jog is generated by setting the free mode trajectory
planner's target position to a point beyond the end of the joint's
range of travel. This ensures that the planner will move constantly
until it is stopped by either the joint limits or an ABORT command.
Normally, a GUI sends a JOG_CONT command when the user presses a jog
button, and ABORT when the button is released.

==== Requirements

The command handler will reject the JOG_CONT command with an error
message if machine is not in free mode, or if any joint is in motion
(GET_MOTION_INPOS_FLAG() == FALSE), or if motion is not enabled. It
will also silently ignore the command if the joint is already at or
beyond its limit and the commanded jog would make it worse.

==== Results

The free mode trajectory planner for the joint identified by
emcmotCommand\->axis is activated, with a target position beyond the end
of joint travel, and a velocity limit of emcmotCommand\->vel. This
starts the joint moving, and the move will continue until stopped by an
ABORT command or by hitting a limit. The free mode planner accelerates
at the joint accel limit at the beginning of the move, and will
decelerate at the joint accel limit when it stops.

=== JOG_INCR

The JOG_INCR command initiates an incremental jog on a single joint.
Incremental jogs are cumulative, in other words, issuing two JOG_INCR
commands that each ask for 0.100 inches of movement will result in
0.200 inches of travel, even if the second command is issued before the
first one finishes. Normally incremental jogs stop when they have
traveled the desired distance, however they also stop when they hit a
limit, or on an ABORT command.

==== Requirements

The command handler will silently reject the JOG_INCR command if
machine is not in free mode, or if any joint is in motion
(GET_MOTION_INPOS_FLAG() == FALSE), or if motion is not enabled. It
will also silently ignore the command if the joint is already at or
beyond its limit and the commanded jog would make it worse.

==== Results

The free mode trajectory planner for the joint identified by
emcmotCommand\->axis is activated, the target position is
incremented/decremented by emcmotCommand\->offset, and the velocity
limit is set to emcmotCommand\->vel. The free mode trajectory planner
will generate a smooth trapezoidal move from the present position to
the target position. The planner can correctly handle changes in the
target position that happen while the move is in progress, so multiple
JOG_INCR commands can be issued in quick succession. The free mode
planner accelerates at the joint accel limit at the beginning of the
move, and will decelerate at the joint accel limit to stop at the
target position.

=== JOG_ABS

The JOG_ABS command initiates an absolute jog on a single joint. An
absolute jog is a simple move to a specific location, in joint
coordinates. Normally absolute jogs stop when they reach the desired
location, however they also stop when they hit a limit, or on an ABORT
command.

==== Requirements

The command handler will silently reject the JOG_ABS command if
machine is not in free mode, or if any joint is in motion
(GET_MOTION_INPOS_FLAG() == FALSE), or if motion is not enabled. It
will also silently ignore the command if the joint is already at or
beyond its limit and the commanded jog would make it worse.

==== Results

The free mode trajectory planner for the joint identified by
emcmotCommand\->axis is activated, the target position is set to
emcmotCommand\->offset, and the velocity limit is set to
emcmotCommand\->vel. The free mode trajectory planner will generate a
smooth trapezoidal move from the present position to the target
position. The planner can correctly handle changes in the target
position that happen while the move is in progress. If multiple JOG_ABS
commands are issued in quick succession, each new command changes the
target position and the machine goes to the final commanded position.
The free mode planner accelerates at the joint accel limit at the
beginning of the move, and will decelerate at the joint accel limit to
stop at the target position.

=== SET_LINE

The SET_LINE command adds a straight line to the trajectory planner
queue.

(More later)

=== SET_CIRCLE

The SET_CIRCLE command adds a circular move to the trajectory planner
queue.

(More later)

=== SET_TELEOP_VECTOR

The SET_TELEOP_VECTOR command instructs the motion controller to move
along a specific vector in Cartesian space.

(More later)

=== PROBE

The PROBE command instructs the motion controller to move toward a
specific point in Cartesian space, stopping and recording its
position if the probe input is triggered.

(More later)

=== CLEAR_PROBE_FLAG

The CLEAR_PROBE_FLAG command is used to reset the probe input in
preparation for a PROBE command. (Question: why shouldn't the PROBE
command automatically reset the input?)

(More later)

=== SET_xix

There are approximately 15 SET_xxx commands, where xxx is the name of
some configuration parameter. It is anticipated that there will be
several more SET commands as more parameters are added. I would like to
find a cleaner way of setting and reading configuration parameters. The
existing methods require many lines of code to be added to multiple
files each time a parameter is added. Much of that code is identical or
nearly identical for every parameter.

== Backlash and Screw Error Compensation

 + FIXME Backlash and Screw Error Compensation

== Task controller (EMCTASK)

=== State

Task has three possible internal states: *E-stop*, *E-stop Reset*,
and *Machine On*.

image::task-state-transitions.svg[align="center"]

== IO controller (EMCIO)

The I/O Controller is separate module that accepts NML commands from TASK.
It interacts with external I/O using HAL pins.
iocontrol.cc is loaded via the linuxcnc script before TASK is.
There are currently two versions of iocontrol. The second version handles toolchange hardware errors.

Currently ESTOP/Enable, coolant, lube, and tool changing are handled by
iocontrol. These are relatively low speed events, high speed coordinated I/O is handled in motion.

emctaskmain.cc sends I/O commands via taskclass.cc.
Taskclass's functions send NML messages out to iocontrol.cc.
Taskclass either uses the commands defined in c++ in it's file or,
if defined, runs python based commands defined in files provided by the user.

iocontrol main loop process:

- registers for SIGTERM and SIGINT signals from the OS.
- checks to see it HAL inputs have changed
- checks if read_tool_inputs() indicates the tool change is finished and set emcioStatus.status
- checks for I/O related NML messages

nml message numbers: from emc.hh:

[source,c]
----
#define EMC_IO_INIT_TYPE                             ((NMLTYPE) 1601)
#define EMC_TOOL_STAT_TYPE                           ((NMLTYPE) 1199)
#define EMC_TOOL_INIT_TYPE                           ((NMLTYPE) 1101)
#define EMC_TOOL_HALT_TYPE                           ((NMLTYPE) 1102)
#define EMC_TOOL_ABORT_TYPE                          ((NMLTYPE) 1103)
#define EMC_TOOL_PREPARE_TYPE                        ((NMLTYPE) 1104)
#define EMC_TOOL_LOAD_TYPE                           ((NMLTYPE) 1105)
#define EMC_TOOL_UNLOAD_TYPE                         ((NMLTYPE) 1106)
#define EMC_TOOL_LOAD_TOOL_TABLE_TYPE                ((NMLTYPE) 1107)
#define EMC_TOOL_SET_OFFSET_TYPE                     ((NMLTYPE) 1108)
#define EMC_TOOL_SET_NUMBER_TYPE                     ((NMLTYPE) 1109)
// the following message is sent to io at the very start of an M6
// even before emccanon issues the move to toolchange position
#define EMC_TOOL_START_CHANGE_TYPE                   ((NMLTYPE) 1110)
----

== User Interfaces

  FIXME User Interfaces

== libnml Introduction

libnml is derived from the NIST rcslib without all the multi-platform
support. Many of the wrappers around platform specific code has been
removed along with much of the code that is not required by LinuxCNC. It is
hoped that sufficient compatibility remains with rcslib so that
applications can be implemented on non-Linux platforms and still be
able to communicate with LinuxCNC.

This chapter is not intended to be a definitive guide to using libnml
(or rcslib), instead, it will eventually provide an overview of each
C++ class and their member functions. Initially, most of these notes
will be random comments added as the code scrutinized and modified.

== LinkedList

Base class to maintain a linked list. This is one of the core building
blocks used in passing NML messages and assorted internal data
structures.

== LinkedListNode

Base class for producing a linked list - Purpose, to hold pointers to
the previous and next nodes, pointer to the data, and the size of the
data.

No memory for data storage is allocated.

== SharedMemory

Provides a block of shared memory along with a semaphore (inherited
from the Semaphore class). Creation and destruction of the semaphore is
handled by the SharedMemory constructor and destructor.

== ShmBuffer

Class for passing NML messages between local processes using a shared
memory buffer. Much of internal workings are inherited from the CMS
class.

== Timer

The Timer class provides a periodic timer limited only by the
resolution of the system clock. If, for example, a process needs to be
run every 5 seconds regardless of the time taken to run the process,
the following code snippet demonstrates how :

[source,c]
----
main()
{
    timer = new Timer(5.0);    /* Initialize a timer with a 5 second loop */
    while(0) {
        /* Do some process */
        timer.wait();    /* Wait till the next 5 second interval */
    }
    delete timer;
}
----

== Semaphore

The Semaphore class provides a method of mutual exclusions for
accessing a shared resource. The function to get a semaphore can either
block until access is available, return after a timeout, or return
immediately with or without gaining the semaphore. The constructor will
create a semaphore or attach to an existing one if the ID is already in
use.

The Semaphore::destroy() must be called by the last process only.

== CMS

At the heart of libnml is the CMS class, it contains most of the
functions used by libnml and ultimately NML. Many of the internal
functions are overloaded to allow for specific hardware dependent
methods of data passing. Ultimately, everything revolves around a
central block of memory (referred to as the 'message buffer' or just
'buffer'). This buffer may exist as a shared memory block accessed by
other CMS/NML processes, or a local and private buffer for data being
transferred by network or serial interfaces.

The buffer is dynamically allocated at run time to allow for greater
flexibility of the CMS/NML sub-system. The buffer size must be large
enough to accommodate the largest message, a small amount for internal
use and allow for the message to be encoded if this option is chosen
(encoded data will be covered later). The following figure is an
internal view of the buffer space.

image::CMS_buffer.png[align="center"]

.CMS buffer
The CMS base class is primarily responsible for creating the
communications pathways and interfacing to the operating system.

////////////////////////////////////////////////////////////////////////
== NML Notes /* FIX ME */

A collection of random notes and thought whilst studying the libnml
and rcslib code.

Much of this needs to be edited and re-written in a coherent manner
before publication.
////////////////////////////////////////////////////////////////////////

== Configuration file format

NML configuration consists of two types of line formats. One for
Buffers, and a second for Processes that connect to the buffers.

=== Buffer line

The original NIST format of the buffer line is:

* 'B name type host size neut RPC# buffer# max_procs key [type specific configs]'
* 'B' - identifies this line as a Buffer configuration.
* 'name' - is the identifier of the buffer.
* 'type' - describes the buffer type - SHMEM, LOCMEM, FILEMEM, PHANTOM, or GLOBMEM.
* 'host' - is either an IP address or host name for the NML server
* 'size' - is the size of the buffer
* 'neut' - a boolean to indicate if the data in the buffer is encoded in a
  machine independent format, or raw.
* 'RPC#' - Obsolete - Place holder retained for backward compatibility only.
* 'buffer#' - A unique ID number used if a server controls multiple buffers.
* 'max_procs' - is the maximum processes allowed to connect to this buffer.
* 'key' - is a numerical identifier for a shared memory buffer

=== Type specific configs

The buffer type implies additional configuration options whilst the
host operating system precludes certain combinations. In an attempt to
distill published documentation in to a coherent format, only the *SHMEM*
buffer type will be covered.

* 'mutex=os_sem' - default mode for providing semaphore locking of the buffer memory.
* 'mutex=none' - Not used
* 'mutex=no_interrupts' - not applicable on a Linux system
* 'mutex=no_switching' - not applicable on a Linux system
* 'mutex=mao split' - Splits the buffer in to half (or more) and allows
  one process to access part of the buffer whilst a second process is
  writing to another part.
* 'TCP=(port number)' - Specifies which network port to use.
* 'UDP=(port number)' - ditto
* 'STCP=(port number)' - ditto
* 'serialPortDevName=(serial port)' - Undocumented.
* 'passwd=file_name.pwd' - Adds a layer of security to the buffer by
  requiring each process to provide a password.
* 'bsem' - NIST documentation implies a key for a blocking semaphore,
  and if bsem=-1, blocking reads are prevented.
* 'queue' - Enables queued message passing.
* 'ascii' - Encode messages in a plain text format
* 'disp' - Encode messages in a format suitable for display (???)
* 'xdr' - Encode messages in External Data Representation. (see rpc/xdr.h for details).
* 'diag' - Enables diagnostics stored in the buffer (timings and byte counts ?)

=== Process line

The original NIST format of the process line is:

*P name buffer type host ops server timeout master c_num [type specific configs]*

* 'P' - identifies this line as a Process configuration.
* 'name' - is the identifier of the process.
* 'buffer' - is one of the buffers defined elsewhere in the config file.
* 'type' - defines whether this process is local or remote relative to the buffer.
* 'host' - specifies where on the network this process is running.
* 'ops' - gives the process read only, write only, or read/write access to the buffer.
* 'server' - specifies if this process will running a server for this buffer.
* 'timeout' - sets the timeout characteristics for accesses to the buffer.
* 'master' - indicates if this process is responsible for creating and destroying the buffer.
* 'c_num' - an integer between zero and (max_procs -1)

=== Configuration Comments

Some of the configuration combinations are invalid, whilst others
imply certain constraints. On a Linux system, GLOBMEM is obsolete,
whilst PHANTOM is only really useful in the testing stage of an
application, likewise for FILEMEM. LOCMEM is of little use for a
multi-process application, and only offers limited performance
advantages over SHMEM. This leaves SHMEM as the only buffer type to use
with LinuxCNC.

The neut option is only of use in a multi-processor system where
different (and incompatible) architectures are sharing a block of
memory. The likelihood of seeing a system of this type outside of a
museum or research establishment is remote and is only relevant to
GLOBMEM buffers.

The RPC number is documented as being obsolete and is retained only
for compatibility reasons.

With a unique buffer name, having a numerical identity seems to be
pointless. Need to review the code to identify the logic. Likewise, the
key field at first appears to be redundant, and it could be derived
from the buffer name.

The purpose of limiting the number of processes allowed to connect to
any one buffer is unclear from existing documentation and from the
original source code. Allowing unspecified multiple processes to
connect to a buffer is no more difficult to implement.

The mutex types boil down to one of two, the default "os_sem" or "mao
split". Most of the NML messages are relatively short and can be copied
to or from the buffer with minimal delays, so split reads are not
essential.

Data encoding is only relevant when transmitted to a remote process -
Using TCP or UDP implies XDR encoding. Whilst ASCII encoding may have
some use in diagnostics or for passing data to an embedded system that
does not implement NML.

UDP protocols have fewer checks on data and allows a percentage of
packets to be dropped. TCP is more reliable, but is marginally slower.

If LinuxCNC is to be connected to a network, one would hope that it is
local and behind a firewall. About the only reason to allow access to
LinuxCNC via the Internet would be for remote diagnostics - This can be
achieved far more securely using other means, perhaps by a web
interface.

The exact behavior when timeout is set to zero or a negative value is
unclear from the NIST documents. Only INF and positive values are
mentioned. However, buried in the source code of rcslib, it is apparent
that the following applies:

timeout > 0 Blocking access until the timeout interval is reached or
access to the buffer is available.

timeout = 0 Access to the buffer is only possible if no other process
is reading or writing at the time.

timeout < 0 or INF Access is blocked until the buffer is available.

== NML base class

// FIXME

Expand on the lists and the relationship between NML, NMLmsg, and the
lower level cms classes.

Not to be confused with NMLmsg, RCS_STAT_MSG, or RCS_CMD_MSG.

NML is responsible for parsing the config file, configuring the cms
buffers and is the mechanism for routing messages to the correct
buffer(s). To do this, NML creates several lists for:

* cms buffers created or connected to.
* processes and the buffers they connect to
* a long list of format functions for each message type

This last item is probably the nub of much of the malignment of
libnml/rcslib and NML in general. Each message that is passed via NML
requires a certain amount of information to be attached in addition to
the actual data. To do this, several formatting functions are called in
sequence to assemble fragments of the overall message. The format
functions will include NML_TYPE, MSG_TYPE, in addition to the data
declared in derived NMLmsg classes. Changes to the order in which the
formatting functions are called and also the variables passed will
break compatibility with rcslib if messed with - There are reasons for
maintaining rcslib compatibility, and good reasons for messing with the
code. The question is, which set of reasons are overriding?

=== NML internals

==== NML constructor

NML::NML() parses the config file and stores it in a linked list to be
passed to cms constructors in single lines. It is the function of the
NML constructor to call the relevant cms constructor for each buffer
and maintain a list of the cms objects and the processes associated
with each buffer.

It is from the pointers stored in the lists that NML can interact with
cms and why Doxygen fails to show the real relationships involved.

[NOTE]
The config is stored in memory before passing a pointer to
a specific line to the cms constructor. The cms constructor then parses
the line again to extract a couple of variables... It would make more
sense to do ALL the parsing and save the variables in a struct that is
passed to the cms constructor - This would eliminate string handling
and reduce duplicate code in cms...

==== NML read/write

Calls to NML::read and NML::write both perform similar tasks in so
much as processing the message - The only real variation is in the
direction of data flow.

A call to the read function first gets data from the buffer, then
calls format_output(), whilst a write function would call
format_input() before passing the data to the buffer. It is in
format_xxx() that the work of constructing or deconstructing the
message takes place. A list of assorted functions are called in turn to
place various parts of the NML header (not to be confused with the cms
header) in the right order - The last function called is emcFormat() in
emc.cc.

==== NMLmsg and NML relationships

NMLmsg is the base class from which all message classes are derived.
Each message class must have a unique ID defined (and passed to the
constructor) and also an update(*cms) function. The update() will be
called by the NML read/write functions when the NML formatter is called --
the pointer to the formatter will have been declared in the NML
constructor at some point. By virtue of the linked lists NML creates,
it is able to select cms pointer that is passed to the formatter and
therefor which buffer is to be used.

== Adding custom NML commands

LinuxCNC is pretty awesome, but some parts need some tweaking. As you know
communication is done through NML channels, the data sent through such
a channel is one of the classes defined in emc.hh (implemented in
emc.cc). If somebody needs a message type that doesn't exist, he should
follow these steps to add a new one. (The Message I added in the
example is called EMC_IO_GENERIC (inherits EMC_IO_CMD_MSG (inherits
RCS_CMD_MSG)))

. add the definition of the EMC_IO_GENERIC class to emc2/src/emc/nml_intf/emc.hh
. add the type define: #define EMC_IO_GENERIC_TYPE ((NMLTYPE) 1605) +
.. (I chose 1605, because it was available) to emc2/src/emc/nml_intf/emc.hh
. add case EMC_IO_GENERIC_TYPE to emcFormat in emc2/src/emc/nml_intf/emc.cc
. add case EMC_IO_GENERIC_TYPE to emc_symbol_lookup in emc2/src/emc/nml_intf/emc.cc
. add EMC_IO_GENERIC::update function to emc2/src/emc/nml_intf/emc.cc

Recompile, and the new message should be there. The next part is to
send such messages from somewhere, and receive them in another place,
and do some stuff with it.

== The Tool Table and Toolchanger

LinuxCNC interfaces with toolchanger hardware, and has an internal
toolchanger abstraction.  LinuxCNC manages tool information in a tool
table file.

=== Toolchanger abstraction in LinuxCNC

LinuxCNC supports two kinds of toolchanger hardware,
called _nonrandom_ and _random_.  The INI setting
<<sub:ini:sec:emcio,[EMCIO]RANDOM_TOOLCHANGER>> controls which of
these kinds of hardware LinuxCNC thinks it's connected to.

==== Nonrandom Toolchangers

Nonrandom toolchanger hardware puts each tool back in the pocket it was
originally loaded from.

Examples of nonrandom toolchanger hardware are the "manual" toolchanger,
lathe tool turrents, and rack toolchangers.

When configured for a nonrandom toolchanger, LinuxCNC does not change the
pocket number in the tool table file as tools are loaded and unloaded.
Internal to LinuxCNC, on tool change the tool information is *copied*
from the tool table's source pocket to pocket 0 (which represents the
spindle), replacing whatever tool information was previously there.

[NOTE]
In LinuxCNC configured for nonrandom toolchanger, tool 0 (T0) has
special meaning: "no tool".  T0 may not appear in the tool table file, and
changing to T0 will result in LinuxCNC thinking it's got an empty spindle.

==== Random Toolchangers

Random toolchanger hardware swaps the tool in the spindle (if any) with
the requested tool on tool change.  Thus the pocket that a tool resides
in changes as it is swapped in and out of the spindle.

An example of random toolchanger hardware is a carousel toolchanger.

When configured for a random toolchanger, LinuxCNC swaps the pocket number
of the old and the new tool in the tool table file when tools are loaded.
Internal to LinuxCNC, on tool change, the tool information is *swapped*
between the tool table's source pocket and pocket 0 (which represents
the spindle).  So after a tool change, pocket 0 in the tool table has
the tool information for the new tool, and the pocket that the new tool
came from has the tool information for the old tool (the tool that was
in the spindle before the tool change), if any.

[NOTE]
If LinuxCNC is configured for random toolchanger, tool 0 (T0) has *no*
special meaning.  It is treated exactly like any other tool in the tool
table.  It is customary to use T0 to represent "no tool" (ie, a tool with
zero TLO), so that the spindle can be conveniently emptied when needed.

=== The Tool Table

LinuxCNC keeps track of tools in a file called the <<sec:tool-table,tool
table>>.  The tool table records the following information for each tool:

tool number::
  An integer that uniquely identifies this tool.  Tool numbers are
  handled differently by LinuxCNC when configured for random and
  nonrandom toolchangers:
+
  * When LinuxCNC is configured for a nonrandom toolchanger this
    number must be positive.  T0 gets special handling and is not
    allowed to appear in the tool table.
  * When LinuxCNC is configured for a random toolchanger this number
    must be non-negative.  T0 is allowed in the tool table, and is
    usually used to represent "no tool", i.e. the empty pocket.

pocket number::
  An integer that identifies the pocket or slot in the toolchanger
  hardware where the tool resides.  Pocket numbers are handled
  differently by LinuxCNC when configured for random and nonrandom
  toolchangers:
+
  * When LinuxCNC is configured for a nonrandom toolchanger, the pocket
    number in the tool file can be any positive integer (pocket
    0 is not allowed).  LinuxCNC silently compactifies the pocket
    numbers when it loads the tool file, so there may be a difference
    between the pocket numbers in the tool file and the internal
    pocket numbers used by LinuxCNC-with-nonrandom-toolchanger.
  * When LinuxCNC is configured for a random toolchanger, the pocket
    numbers in the tool file must be between 0 and 1000, inclusive.
    Pockets 1-1000 are in the toolchanger, pocket 0 is the spindle.

diameter::
  Diameter of the tool, in machine units.

tool length offset::
  Tool length offset (also called TLO), in up to 9 axes, in machine
  units.  Axes that don't have a specified TLO get 0.

=== G-codes affecting tools

The G-codes that use or affect tool information are:

==== Txxx

Tells the toolchanger hardware to prepare to switch to a specified
tool +xxx+.

Handled by +Interp::convert_tool_select()+.

. The machine is asked to prepare to switch to the selected tool by
  calling the Canon function +SELECT_TOOL()+ with the tool number
  of the requested tool.
.. (saicanon) No-op.
.. (emccanon) Builds an +EMC_TOOL_PREPARE+ message with the requested
   pocket number and sends it to Task, which sends it on
   to IO.  IO gets the message and asks HAL to prepare
   the pocket by setting +iocontrol.0.tool-prep-pocket+,
   +iocontrol.0.tool-prep-number+, and +iocontrol.0.tool-prepare+.
   IO then repeatedly calls +read_tool_inputs()+ to poll the HAL pin
   +iocontrol.0.tool-prepared+, which signals from the toolchanger
   hardware, via HAL, to IO that the requested tool prep is complete.
   When that pin goes True, IO sets +emcioStatus.tool.pocketPrepped+
   to the requested tool's pocket number.
. Back in interp, +settings->selected_pocket+ is assigned the tooldata
  index of the requested tool _xxx_.

[NOTE]
The legacy names *selected_pocket* and *current_pocket* actually reference
a sequential tooldata index for tool items loaded from a tool
table ([EMCIO]TOOL_TABLE) or via a tooldata database ([EMCIO]DB_PROGRAM).

==== M6

Tells the toolchanger to switch to the currently selected tool (selected
by the previous Txxx command).

Handled by +Interp::convert_tool_change()+.

. The machine is asked to change to the selected tool
  by calling the Canon function +CHANGE_TOOL()+ with
  +settings->selected_pocket+ (a tooldata index).
.. (saicanon) Sets sai's +_active_slot+ to the passed-in pocket
   number.  Tool information is copied from the selected pocket
   of of the tool table (ie, from sai's +_tools[_active_slot]+)
   to the spindle (aka sai's +_tools[0]+).
.. (emccanon) Sends an +EMC_TOOL_LOAD+ message to Task, which
   sends it to IO.  IO sets +emcioStatus.tool.toolInSpindle+
   to the tool number of the tool in the pocket identified
   by +emcioStatus.tool.pocketPrepped+ (set by +Txxx+
   aka +SELECT_TOOL()+).  It then requests that the
   toolchanger hardware perform a tool change, by setting
   the HAL pin +iocontrol.0.tool-change+ to True.  Later,
   IO's +read_tool_inputs()+ will sense that the HAL pin
   +iocontrol.0.tool_changed+ has been set to True, indicating the
   toolchanger has completed the tool change.  When this happens,
   it calls +load_tool()+ to update the machine state.
... +load_tool()+ with a nonrandom toolchanger
    config copies the tool information from the selected pocket
    to the spindle (pocket 0).
... +load_tool()+ with a random toolchanger config swaps tool
    information between pocket 0 (the spindle) and the selected
    pocket, then saves the tool table.
. Back in interp, +settings->current_pocket+ is assigned the new
  tooldata index from +settings->selected_pocket+ (set by +Txxx+).  The relevant
  numbered parameters (<<sub:numbered-parameters,#5400-#5413>>) are
  updated with the new tool information from pocket 0 (spindle).

==== G43/G43.1/G49

Apply tool length offset.  G43 uses the TLO of the currently loaded tool,
or of a specified tool if the H-word is given in the block.  G43.1 gets
TLO from axis-words in the block.  G49 cancels the TLO (it uses 0 for
the offset for all axes).

Handled by +Interp::convert_tool_length_offset()+.

. It starts by building an +EmcPose+ containing the 9-axis offsets
  to use.  For +G43.1+, these tool offsets come from axis words in the
  current block.  For +G43+ these offsets come from the current tool
  (the tool in pocket 0), or from the tool specified by the H-word in
  the block.  For G49, the offsets are all 0.
. The offsets are passed to Canon's +USE_TOOL_LENGTH_OFFSET()+ function.
.. (saicanon) Records the TLO in +_tool_offset+.
.. (emccanon) Builds an +EMC_TRAJ_SET_OFFSET+ message containing the
   offsets and sends it to Task.  Task copies the offsets to
   +emcStatus->task.toolOffset+ and sends them on to Motion via
   an +EMCMOT_SET_OFFSET+ command.  Motion copies the offsets
   to +emcmotStatus->tool_offset+, where it gets used to offset
   future motions.
. Back in interp, the offsets are recorded in +settings->tool_offset+.
  The effective pocket is recorded in +settings->tool_offset_index+,
  though this value is never used.

==== G10 L1/L10/L11

Modifies the tool table.

Handled by +Interp::convert_setup_tool()+.

. Picks the tool number out of the P-word in the block and finds the
  pocket for that tool:
.. With a nonrandom toolchanger config this is always the
   pocket number in the toolchanger (even when the tool is in
   the spindle).
.. With a random toolchanger config, if the tool is currently
   loaded it uses pocket 0 (pocket 0 means "the spindle"),
   and if the tool is not loaded it uses the pocket number in
   the tool changer.  (This difference is important.)
. Figures out what the new offsets should be.
. The new tool information (diameter, offsets, angles, and orientation),
  along with the tool number and pocket number, are passed to the Canon
  call SET_TOOL_TABLE_ENTRY().
.. (saicanon)  Copy the new tool information to the specified pocket
   (in sai's internal tool table, +_tools+).
.. (emccanon)  Build an +EMC_TOOL_SET_OFFSET+ message with the new
   tool information, and send it to Task, which passes it
   to IO.  IO updates the specified pocket in its internal
   copy of the tool table (+emcioStatus.tool.toolTable+), and
   if the specified tool is currently loaded (it is compared to
   +emcioStatus.tool.toolInSpindle+) then the new tool information
   is copied to pocket 0 (the spindle) as well.  (FIXME: that's a
   buglet, should only be copied on nonrandom machines.)  Finally IO
   saves the new tool table.
. Back in interp, if the modified tool is currently loaded in the
  spindle, and if the machine is a non-random toolchanger, then
  the new tool information is copied from the tool's home pocket
  to pocket 0 (the spindle) in interp's copy of the tool table,
  +settings->tool_table+.  (This copy is not needed on random tool
  changer machines because there, tools don't have a home pocket and
  instead we just updated the tool in pocket 0 directly.)
. The relevant numbered parameters
  (<<sub:numbered-parameters,#5400-#5413>>) are updated from the tool
  information in the spindle (by copying the information from interp's
  +settings->tool_table+ to +settings->parameters+).  (FIXME: this is
  a buglet, the params should only be updated if it was the current
  tool that was modified).
. If the modified tool is currently loaded in the
  spindle, and if the config is for a nonrandom toolchanger, then the
  new tool information is written to the tool table's pocket 0 as well,
  via a second call to SET_TOOL_TABLE_ENTRY().  (This second tool-table
  update is not needed on random toolchanger machines because there,
  tools don't have a home pocket and instead we just updated the tool
  in pocket 0 directly.)

==== M61

Set current tool number.  This switches LinuxCNC's internal representation
of which tool is in the spindle, without actually moving the toolchanger
or swapping any tools.

Handled by +Interp::convert_tool_change()+.

Canon: +CHANGE_TOOL_NUMBER()+

settings->current_pocket is assigned the tooldata index currently
holding the tool specified by the Q-word argument.

==== G41/G41.1/G42/G42.1

Enable cutter radius compensation (usually called _cutter comp_).

Handled by +Interp::convert_cutter_compensation_on()+.

No Canon call, cutter comp happens in the interpreter.  Uses the tool
table in the expected way: if a D-word tool number is supplied it looks
up the pocket number of the specified tool number in the table, and if
no D-word is supplied it uses pocket 0 (the spindle).

==== G40

Cancel cutter radius compensation.

Handled by +Interp::convert_cutter_compensation_off()+.

No Canon call, cutter comp happens in the interpreter.  Does not use
the tool table.

=== Internal state variables

This is not an exhaustive list!  Tool information is spread through
out LinuxCNC.

==== IO

+emcioStatus+ is of type +EMC_IO_STAT+

emcioStatus.tool.pocketPrepped::
  When IO gets the signal from HAL that the toolchanger prep is
  complete (after a +Txxx+ command), this variable is set to the
  pocket of the requested tool.  When IO gets the signal from HAL
  that the tool change itself is complete (after an +M6+ command),
  this variable gets reset to -1.

emcioStatus.tool.toolInSpindle::
  Tool number of the tool currently installed in the spindle.
  Exported on the HAL pin +iocontrol.0.tool-number+ (s32).

emcioStatus.tool.toolTable[]::
  An array of +CANON_TOOL_TABLE+ structures, +CANON_POCKETS_MAX+ long.
  Loaded from the tool table file at startup and maintained there
  after.  Index 0 is the spindle, indexes 1-(CANON_POCKETS_MAX-1)
  are the pockets in the toolchanger.  This is a complete copy
  of the tool information, maintained separately from Interp's
  +settings.tool_table+.

==== interp

+settings+ is of type +settings+, defined as +struct setup_struct+
in +src/emc/rs274ngc/interp_internal.hh+.

settings.selected_pocket::
  Tooldata index of the tool most recently selected by +Txxx+.

settings.current_pocket::
  Original tooldata index of the tool currently in the spindle.  In other words:
  which tooldata index the tool that's currently in the spindle
  was loaded from.

settings.tool_table[]::
  An array of tool information.  The index into the array is the "pocket
  number" (aka "slot number").  Pocket 0 is the spindle, pockets 1
  through (CANON_POCKETS_MAX-1) are the pockets of the toolchanger.

settings.tool_offset_index::
  Unused.  FIXME: Should probably be removed.

settings.toolchange_flag::
  Interp sets this to true when calling Canon's CHANGE_TOOL()
  function.  It is checked in +Interp::convert_tool_length_offset()+
  to decide which tooldata index to use for G43 (with no H-word):
  +settings->current_pocket+ if the tool change is still in progress,
  tooldata index 0 (the spindle) if the tool change is complete.

settings.random_toolchanger::
  Set from the INI variable +[EMCIO]RANDOM_TOOLCHANGER+ at startup.
  Controls various tool table handling logic.  (IO also reads this
  INI variable and changes its behavior based on it.  For example,
  when saving the tool table, random toolchanger save the tool in
  the spindle (pocket 0), but non-random toolchanger save each tool
  in its "home pocket".)

settings.tool_offset::
  This is an +EmcPose+ variable.
+
* Used to compute position in various places.
* Sent to Motion via the +EMCMOT_SET_OFFSET+ message.
  All motion does with the offsets is export them to the HAL
  pins +motion.0.tooloffset.[xyzabcuvw]+.  FIXME: export these from
  someplace closer to the tool table (io or interp, probably)
  and remove the EMCMOT_SET_OFFSET message.

settings.pockets_max::
  Used interchangeably with +CANON_POCKETS_MAX+ (a #defined constant,
  set to 1000 as of April 2020).  FIXME: This settings variable
  is not currently useful and should probably be removed.

settings.tool_table::
  This is an array of +CANON_TOOL_TABLE+ structures (defined in
  +src/emc/nml_intf/emctool.h+), with +CANON_POCKETS_MAX+ entries.
  Indexed by "pocket number", aka "slot number".  Index 0 is the
  spindle, indexes 1 to (CANON_POCKETS_MAX-1) are the pockets in the tool
  changer.  On a random toolchanger pocket numbers are meaningful.
  On a nonrandom toolchanger pockets are meaningless; the pocket
  numbers in the tool table file are ignored and tools are assigned
  to +tool_table+ slots sequentially.

settings.tool_change_at_g30::
settings.tool_change_quill_up::
settings.tool_change_with_spindle_on::
  These are set from INI variables in the +[EMCIO]+ section, and
  determine how tool changes are performed.

== Reckoning of joints and axes

=== In the status buffer

The status buffer is used by Task and the UIs.

FIXME: `axis_mask` and `axes` overspecify the number of axes

`status.motion.traj.axis_mask`::
  A bitmask with a "1" for the axes that are present and a "0"
  for the axes that are not present. X is bit 0 with value 2^0^ = 1 if set, Y is bit 1 with value 2^1^ = 2, Z is bit 2 with value 4, etc.
  For example, a machine with X and Z axes would have an `axis_mask`
  of 0x5, an XYZ machine would have 0x7, and an XYZB machine would
  have an `axis_mask` of 0x17.

`status.motion.traj.axes` (removed)::
  This value was removed in LinuxCNC version 2.9.  Use `axis_mask` instead.

`status.motion.traj.joints`::
  A count of the number of joints the machine has. A normal lathe
  has 2 joints; one driving the X axis and one driving the Z axis.
  An XYYZ gantry mill has 4 joints: one driving X, one driving one side
  of the Y, one driving the other side of the Y, and one driving Z.
  An XYZA mill also has 4 joints.

`status.motion.axis[EMCMOT_MAX_AXIS]`::
  An array of `EMCMOT_MAX_AXIS` axis structures. `axis[n]` is valid
  if `(axis_mask & (1 << n))` is True. If `(axis_mask & (1 << n))`
  is False, then `axis[n]` does not exist on this machine and must
  be ignored.

`status.motion.joint[EMCMOT_MAX_JOINTS]`::
  An array of `EMCMOT_MAX_JOINTS` joint structures. `joint[0]` through
  `joint[joints-1]` are valid, the others do not exist on this machine
  and must be ignored.

Things are not this way currently in the joints-axes branch, but
deviations from this design are considered bugs. For an example of such
a bug, see the treatment of axes in src/emc/ini/initraj.cc:loadTraj().
There are undoubtedly more, and I need your help to find them and
fix them.

=== In Motion

The Motion controller realtime component first gets the number of joints
from the `num_joints` load-time parameter.  This determines how many
joints worth of HAL pins are created at startup.

Motion's number of joints can be changed at runtime using the
`EMCMOT_SET_NUM_JOINTS` command from Task.

The Motion controller always operates on `EMCMOT_MAX_AXIS` axes.
It always creates nine sets of `axis.*.*` pins.

// vim: set syntax=asciidoc:
