Developers Handbook

This handbook is a work in progress. If you are able to help with writing, editing, or graphic preparation please contact any member of the writing team or join and send an email to emc-users@lists.sourceforge.net.

Copyright (c) 2000-7 LinuxCNC.org

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and one Back-Cover Text: This EMC Handbook is the product of several authors writing for linuxCNC.org. As you find it to be of value in your work, we invite you to contribute to its revision and growth.A copy of the license is included in the section entitled GNU Free Documentation License. If you do not find the license you may order a copy from Free Software Foundation, Inc. 59 Temple Place, Suite 330 Boston, MA 02111-1307

EMC2 Code Notes

Introduction

Intended audience

This document is a collection of notes about the internals of EMC2. 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 EMC2 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 Much of this information is now outdated and has never been reviewed for accuracy.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 EMC2, as well as chapter(s) covering how they work together. This document is very much a work in progress, and it’s layout may change in the future.

Overview of EMC2

Terms and definitions

AXIS: An axis is one of the six degrees of freedom that define a tool position in three dimensional Cartesian space. Those axes are X, Y, Z, A, B, and C, where X, Y, and Z are linear coordinates that determine where the tip of the tool is, and A, B, and C are angular coordinates that determine the tool orientation. 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 EMC2, 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.
POSE: A pose is a fully specified position in 3-D Cartesian space. In the EMC2 motion controller, when we refer to a pose we mean an EmcPose structure, containing three linear coordinates and three angular ones.

Architecture overview

There are four components contained in the EMC2 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.).

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 basic information about the HAL, read the “Introduction to HAL” document. 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 emc2/src/hal/hal.h.

Block diagrams and Data Flow

Figure [fig:motion-joint-controller-block-diag] 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

Figure [fig:motion-joint-controller-block-diag] 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 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.

home_final_vel

home_final_vel is also a member of the joint structure. It specifies the speed that EMC uses when it makes its move from HOME_OFFSET to the HOME position. If the HOME_FINAL_VEL is missing from the ini file, then the maximum joint speed is used to make this move.

home_ignore_limits

home_ignore_limits is a single bit within the joint structure member home_flags. This flag determines whether EMC will ignore the limit switch inputs. Some machine configurations do not use a separate home switch, instead they route one of the limit switch signals to the home switch input as well. In this case, EMC needs to ignore that limit during homing. The default value for this parameter is OFF.

home_use_index

home_use_index is a single bit within the joint structure member home_flags. It specifies whether or not there is an index pulse. If the flag is true, EMC will latch on the rising edge of the index pulse. If false, EMC will latch on either the rising or falling edge of the home switch (depending on the signs of search_vel and latch_vel). If search_vel is zero, the latch phase is skipped and this parameter is ignored. The default value is OFF.

home_offset

home_offset is a member of the joint structure. It contains the location of the home switch or index pulse, in joint coordinates. It can also be treated as the distance between the point where the switch or index pulse is latched and the zero point of the joint. After detecting the index pulse, EMC sets the joint coordinate of that point to “home_offset”. The default value is zero.

home

home is a member of the joint structure. It is the position that the joint will go to upon completion of the homing sequence. After detecting the index pulse, and setting the coordinate of that point to “home_offset”, EMC makes a move to "home" as the final step of the homing process. The default value is zero. Note that even if this parameter is the same as “home_offset”, the axis will slightly overshoot the latched position as it stops. Therefore there will always be a small move at this time (unless search_vel is zero, and the entire search/latch stage was skipped). This final move will be made at the joint’s maximum velocity. Since the axis is now homed, there should be no risk of crashing the machine, and a rapid move is the quickest way to finish the homing sequence.
[The distinction between home and home_offset is not as clear as I would like. I intend to make a small drawing and example to help clarify it. ]

Backlash and Screw Error Compensation

Task controller (EMCTASK)

IO controller (EMCIO)

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 emc2. 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 emc2.

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 :

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). Figure [fig:CMS buffer] is an internal view of the buffer space.

CMS buffer

The CMS base class is primarily responsible for creating the communications pathways and interfacing to the O.S.

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 emc2.

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 emc2 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 emc2 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 /* FIX ME */

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.

(side 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

EMC2 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.

NML Messages

EMC OPERATOR

EMC_OPERATOR_ERROR_TYPE

Description, NML Type: textual error message to the operator, 11

Written From: emccanon.cc, iosh.cc

Read To: emctaskmain.cc, emcsh.cc

Parameter, Type: [error, char[LINELEN]]

EMC_OPERATOR_TEXT_TYPE

Description, NML Type: textual information message to the operator, 12

Written From: emctaskmain.cc

Read To: emctaskmain.cc, emcsh.cc

Parameter, Type: [text, char[LINELEN]]

EMC_OPERATOR_DISPLAY_TYPE

Description, NML Type: URL or filename of a document to display, 13

Obs: not used, only read

Written From: none

Read To: emctaskmain.cc, emcsh.cc

Parameter, Type: [display, char[LINELEN]]

EMC NULL, SET, DEBUG, & SYSTEM

EMC_NULL_TYPE

Description, NML Type: used to reset serial number to original, 21

Written From: thisQuit (emcsh.cc)

Read To: emctaskmain.cc

Parameter, Type: none

EMC_SET_DEBUG_TYPE

Read To: none

Parameter, Type: [axis (in EMC_AXIS_CMD_MSG), int] [homingVel, double]

Description, NML Type: loads compensation values from a file, 131

Obs: currently usrmotLoadComp if 0’ed in emc2

Written From: sendAxisLoadComp (emcsh.cc)

Read To: emcTaskIssueCommand (emctaskmain.cc) calls emcAxisLoadComp (minimill|bridgeporttaskintf.cc) which calls usrmotLoadComp

Parameter, Type: [axis (in EMC_AXIS_CMD_MSG), int] [file, char[LINELEN]]

EMC_AXIS_ALTER_TYPE

Obs: not used

Written From: none

Read To: none

Parameter, Type: [mode, enum EMC_TRAJ_MODE_ENUM]

Obs: used on shutdown

Written From: sendAbort (emcsh.cc)

Read To: emcTaskIssueCommand (emctaskmain.cc) aborts all

Parameter, Type:

EMC_TASK_SET_MODE_TYPE

Description, NML Type: sets current TASK mode, MANUAL, MDI, AUTO, 504

EMC_TOOL_LOAD_TOOL_TABLE_TYPE

Description, NML Type: loads the tool table, without this tool comp. can’t be made, 1107

Written From: sendLoadToolTable (emcsh.cc)

Read To: none

Parameter, Type: [file, char[LINELEN]]

EMC_TOOL_SET_OFFSET_TYPE

Description, NML Type: , 1108

Written From: none

Read To: none

Parameter, Type: [tool, int] [length, double] [diameter, double]

EMC_TOOL_STAT_TYPE

Description, NML Type: , 1199

Obs: not used

Written From: none

Read To: none

Parameter, Type: [value, int] [polarity, int]

Parameter, Type:

EMC LOG

EMC_LOG_OPEN_TYPE

Description, NML Type: opens the log file, 1904

Obs: not used in emc2, it was used in EMC[1] from emclog.tcl

Written From: sendLogOpen (emcsh.cc)

Read To: emcTaskIssueCommand (emctaskmain.cc) calls emcLogOpen (taskintf.cc) which sends EMCMOT_OPEN_LOG

Parameter, Type: [file, char[LINELEN]] [type, int] [size, int] [skip, int] [which, int] [triggerType, int] [triggerVar, int] [triggerThreshold, double]

EMC_LOG_START_TYPE

Description, NML Type: starts logging, 1905

Obs: not used in emc2, it was used in EMC[1] from emclog.tcl

Written From: sendLogStart (emcsh.cc)

Read To: emcTaskIssueCommand (emctaskmain.cc) calls emcLogStart (taskintf.cc) which sends EMCMOT_START_LOG

Parameter, Type: none

EMC_LOG_STOP_TYPE

Description, NML Type: stops logging, 1906

Obs: not used in emc2, it was used in EMC[1] from emclog.tcl

Written From: sendLogStop (emcsh.cc)

Read To: emcTaskIssueCommand (emctaskmain.cc) calls emcLogStop (taskintf.cc) which sends EMCMOT_STOP_LOG

Parameter, Type: none

EMC_LOG_CLOSE_TYPE

Description, NML Type: closes the log file, 1907

Obs: not used in emc2, it was used in EMC[1] from emclog.tcl

Written From: sendLogClose (emcsh.cc)

Read To: emcTaskIssueCommand (emctaskmain.cc) calls emcLogClose (taskintf.cc) which sends EMCMOT_CLOSE_LOG

Parameter, Type: none

EMC STAT

EMC_STAT_TYPE

Description, NML Type: aggregation of all the status messages, not sent as a message but used as is all over the place, 1999

Written From: none

Read To: none

Parameter, Type: [task, EMC_TASK_STAT] [motion, EMC_MOTION_STAT] [io, EMC_IO_STAT] [logFile, char[LINELEN]] [logType, int] [logSize, int] [logSkip, int] [logOpen, int] [logStarted, int] [logPoints, int]

Coding Style Guide

Coding Style

This chapter describes the source code style prefered by the EMC team.

Do no harm

When making small edits to code in a style different than the one described below, observe the local coding style. Rapid changes from one coding style to another decrease code readability.

Never check in code after running “indent” on it. The whitespace changes introduced by indent make it more difficult to follow the revision history of the file.

Do not use an editor that makes unneeded changes to whitespace (e.g., which replaces 8 spaces with a tabstop on a line not otherwise modified, or word-wraps lines not otherwise modified)

Tab Stops

A tab stop always corresponds to 8 spaces. Do not write code that displays correctly only with a differing tab stop setting.

Indentation

Use 4 spaces per level of indentation. Combining 8 spaces into one tab is acceptable but not required.

Placing Braces

Put the opening brace last on the line, and put the closing brace first:

if(x) {

The closing brace is on a line of its own, except in the cases where it is followed by a continuation of the same statement, i.e. a "while" in a do-statement or an "else" in an if-statement, like this:

do {

and

if(x == y) {

This brace-placement also minimizes the number of empty (or almost empty) lines, which allows a greater amount of code or comments to be visible at once in a terminal of a fixed size.

Naming

C is a Spartan language, and so should your naming be. Unlike Modula-2 and Pascal programmers, C programmers do not use cute names like ThisVariableIsATemporaryCounter. A C programmer would call that variable "tmp", which is much easier to write, and not the least more difficult to understand.

However, descriptive names for global variables are a must. To call a global function "foo" is a shooting offense.

GLOBAL variables (to be used only if you really need them) need to have descriptive names, as do global functions. If you have a function that counts the number of active users, you should call that "count_active_users()" or similar, you should not call it "cntusr()".

Encoding the type of a function into the name (so-called Hungarian notation) is brain damaged - the compiler knows the types anyway and can check those, and it only confuses the programmer. No wonder Microsoft makes buggy programs.

LOCAL variable names should be short, and to the point. If you have some random integer loop counter, it should probably be called "i". Calling it "loop_counter" is non-productive, if there is no chance of it being mis-understood. Similarly, "tmp" can be just about any type of variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another problem, which is called the function-growth-hormone-imbalance syndrome. See next chapter.

Functions

Functions should be short and sweet, and do just one thing. They should fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the complexity and indentation level of that function. So, if you have a conceptually simple function that is just one long (but simple) case-statement, where you have to do lots of small things for a lot of different cases, it’s OK to have a longer function.

However, if you have a complex function, and you suspect that a less-than-gifted first-year high-school student might not even understand what the function is all about, you should adhere to the maximum limits all the more closely. Use helper functions with descriptive names (you can ask the compiler to in-line them if you think it’s performance-critical, and it will probably do a better job of it that you would have done).

Another measure of the function is the number of local variables. They shouldn’t exceed 5-10, or you’re doing something wrong. Re-think the function, and split it into smaller pieces. A human brain can generally easily keep track of about 7 different things, anything more and it gets confused. You know you’re brilliant, but maybe you’d like to understand what you did 2 weeks from now.

Commenting

Comments are good, but there is also a danger of over-commenting. NEVER try to explain HOW your code works in a comment: it’s much better to write the code so that the working is obvious, and it’s a waste of time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW. A boxed comment describing the function, return value, and who calls it placed above the body is good. Also, try to avoid putting comments inside a function body: if the function is so complex that you need to separately comment parts of it, you should probably re-read the Functions section again. You can make small comments to note or warn about something particularly clever (or ugly), but try to avoid excess. Instead, put the comments at the head of the function, telling people what it does, and possibly WHY it does it.

If comments along the lines of /* Fix me */ are used, please, please, say why something needs fixing. When a change has been made to the affected portion of code, either remove the comment, or amend it to indicate a change has been made and needs testing.

Shell Scripts & Makefiles

Not everyone has the same tools and packages installed. Some people use vi, others emacs - A few even avoid having either package installed, preferring a lightweight text editor such as nano or the one built in to Midnight Commander.

gawk versus mawk - Again, not everyone will have gawk installed, mawk is nearly a tenth of the size and yet conforms to the Posix AWK standard. If some obscure gawk specific command is needed that mawk does not provide, than the script will break for some users. The same would apply to mawk. In short, use the generic awk invocation in preference to gawk or mawk.

C++ Conventions

C++ coding styles are always likely to end up in heated debates (a bit like the emacs versus vi arguments). One thing is certain however, a common style used by everyone working on a project leads to uniform and readable code.

Naming conventions: Constants either from #defines or enumerations should be in upper case through out. Rationale: Makes it easier to spot compile time constants in the source code. e.g. EMC_MESSAGE_TYPE

Classes and Namespaces should capitalize the first letter of each word and avoid underscores. Rationale: Identifies classes, constructors and destructors. e.g. GtkWidget

Methods (or function names) should follow the C recommendations above and should not include the class name. Rationale: Maintains a common style across C and C++ sources. e.g. get_foo_bar()

However, boolean methods are easier to read if they avoid underscores and use an is prefix (not to be confused with methods that manipulate a boolean). Rationale: Identifies the return value as TRUE or FALSE and nothing else. e.g. isOpen, isHomed

Do NOT use "Not" in a boolean name, it leads only leads to confusion when doing logical tests. e.g. isNotOnLimit or is_not_on_limit are BAD.

Variable names should avoid the use of upper case and underscores except for local or private names. The use of global variables should be avoided as much as possible. Rationale: Clarifies which are variables and which are methods. Public: e.g. axislimit Private: e.g. maxvelocity_

Specific method naming conventions

The terms get and set should be used where an attribute is accessed directly. Rationale: Indicates the purpose of the function or method. e.g. get_foo set_bar

For methods involving boolean attributes, set & reset is preferred. Rationale: As above. e.g. set_amp_enable reset_amp_fault

Math intensive methods should use compute as a prefix. Rationale: Shows that it is computationally intensive and will hog the CPU. e.g. compute_PID

Abbreviations in names should be avoided where possible - The exception is for local variable names. Rationale: Clarity of code. e.g. pointer is preferred over ptr compute is preferred over cmp compare is again preferred over cmp.

Enumerates and other constants can be prefixed by a common type name e.g. enum COLOR { COLOR_RED, COLOR_BLUE };

Excessive use of macros and defines should be avoided - Using simple methods or functions is preferred. Rationale: Improves the debugging process.

Include Statements Header files must be included at the top of a source file and not scattered throughout the body. They should be sorted and grouped by their hierarchical position within the system with the low level files included first. Include file paths should NEVER be absolute - Use the compiler -I flag instead. Rationale: Headers may not be in the same place on all systems.

Pointers and references should have their reference symbol next to the variable name rather than the type name. Rationale: Reduces confusion. e.g. float *x or int &i

Implicit tests for zero should not be used except for boolean variables. e.g. if (spindle_speed != 0) NOT if (spindle_speed)

Only loop control statements must be included in a for() construct. e.g. sum = 0; for (i = 0; i < 10; i++) { sum += value[i]; }

NOT for (i = 0, sum =0; i < 10; i++) sum += value[i];

Likewise, executable statements in conditionals must be avoided. e.g. if (fd = open(file_name) is bad.

Complex conditional statements should be avoided - Introduce temporary boolean variables instead.

Parentheses should be used in plenty in mathematical expressions - Do not rely on operator precedence when an extra parentheses would clarify things.

File names: C++ sources and headers use .cc and .hh extension. The use of .c and .h are reserved for plain C. Headers are for class, method, and structure declarations, not code (unless the functions are declared inline).

Python coding standards

Use the PEP 8 style for Python code.

Comp coding standards

In the declaration portion of a .comp file, begin each declaration at the first column. Insert extra blank lines when they help group related items.

In the code portion of a .comp file, follow normal C coding style.

Glossary

A listing of terms and what they mean. Some terms have a general meaning and several additional meanings for users, installers, and developers.

Acme Screw

A type of lead-screw that uses an acme thread form. Acme threads have somewhat lower friction and wear than simple triangular threads, but ball-screws are lower yet. Most manual machine tools use acme lead-screws.

Axis

One of the computer control movable parts of the machine. For a typical vertical mill, the table is the X axis, the saddle is the Y axis, and the quill or knee is the Z axis. Additional linear axes parallel to X, Y, and Z are called U, V, and W respectively. Angular axes like rotary tables are referred to as A, B, and C.

Axis

One of the Graphical User Interfaces available to users of EMC2. Features the modern use of menus and mouse buttons while automating and hiding some of the more traditional EMC2 controls. It is the only open-source interface that displays the entire tool path as soon as a file is opened.

Backlash

The amount of "play" or lost motion that occurs when direction is reversed in a lead screw. or other mechanical motion driving system. It can result from nuts that are loose on leadscrews, slippage in belts, cable slack, "wind-up" in rotary couplings, and other places where the mechanical system is not "tight". Backlash will result in inaccurate motion, or in the case of motion caused by external forces (think cutting tool pulling on the work piece) the result can be broken cutting tools. This can happen because of the sudden increase in chip load on the cutter as the work piece is pulled across the backlash distance by the cutting tool.

Backlash Compensation

- Any technique that attempts to reduce the effect of backlash without actually removing it from the mechanical system. This is typically done in software in the controller. This can correct the final resting place of the part in motion but fails to solve problems related to direction changes while in motion (think circular interpolation) and motion that is caused when external forces (think cutting tool pulling on the work piece) are the source of the motion.

Ball Screw

A type of lead-screw that uses small hardened steel balls between the nut and screw to reduce friction. Ball-screws have very low friction and backlash, but are usually quite expensive.

Ball Nut

A special nut designed for use with a ball-screw. It contains an internal passage to re-circulate the balls from one end of the screw to the other.

CNC

Computer Numerical Control. The general term used to refer to computer control of machinery. Instead of a human operator turning cranks to move a cutting tool, CNC uses a computer and motors to move the tool, based on a part program .

Comp

A tool used to build, compile and install EMC2 HAL components.

Configuration(n)

A directory containing a set of configuration files. Custom configurations are normally saved in the users home/emc2/configs directory. These files include EMC’s traditional INI file and HAL files. A configuration may also contain several general files that describe tools, parameters, and NML connections.

Configuration(v)

The task of setting up EMC2 so that it matches the hardware on a machine tool.

Coordinate Measuring Machine

A Coordinate Measuring Machine is used to make many accurate measurements on parts. These machines can be used to create CAD data for parts where no drawings can be found, when a hand-made prototype needs to be digitized for moldmaking, or to check the accuracy of machined or molded parts.

Display units

The linear and angular units used for onscreen display.

DRO

A Digital Read Out is a device attached to the slides of a machine tool or other device which has parts that move in a precise manner to indicate the current location of the tool with respect to some reference position. Nearly all DRO’s use linear quadrature encoders to pick up position information from the machine.

EDM

EDM is a method of removing metal in hard or difficult to machine or tough metals, or where rotating tools would not be able to produce the desired shape in a cost-effective manner. An excellent example is rectangular punch dies, where sharp internal corners are desired. Milling operations can not give sharp internal corners with finite diameter tools. A wire EDM machine can make internal corners with a radius only slightly larger than the wire’s radius. A sinker EDM cam make corners with a radius only slightly larger than the radius on the corner of the convex EDM electrode.

EMC

The Enhanced Machine Controller. Initially a NIST project. EMC is able to run a wide range of motion devices.

EMCIO

The module within EMC that handles general purpose I/O, unrelated to the actual motion of the axes.

EMCMOT

The module within EMC that handles the actual motion of the cutting tool. It runs as a real-time program and directly controls the motors.

Encoder

A device to measure position. Usually a mechanical-optical device, which outputs a quadrature signal. The signal can be counted by special hardware, or directly by the parport with emc2.

Feed

Relatively slow, controlled motion of the tool used when making a cut.

Feed rate

The speed at which a motion occurs. In manual mode, jog speed can be set from the graphical interface. In auto or mdi mode feed rate is commanded using a (f) word. F10 would mean ten units per minute.

Feedback

A method (e.g., quadrature encoder signals) by which emc receives information about the position of motors

Feed rate Override

A manual, operator controlled change in the rate at which the tool moves while cutting. Often used to allow the operator to adjust for tools that are a little dull, or anything else that requires the feed rate to be “tweaked”.

Floating Point Number

A number that has a decimal point. (12.300) In HAL it is known as float.

G-Code

The generic term used to refer to the most common part programming language. There are several dialects of G-code, EMC uses RS274/NGC.

GUI

Graphical User Interface.

General: A type of interface that allows communications between a computer and human (in most cases) via the manipulation of icons and other elements (widgets) on a computer screen.
EMC: An application that presents a graphical screen to the machine operator allowing manipulation of machine and the corresponding controlling program.

HAL

H*ardware *A*bstraction *L ayer. At the highest level, it is simply a way to allow a number of building blocks to be loaded and interconnected to assemble a complex system. Many of the building blocks are drivers for hardware devices. However, HAL can do more than just configure hardware drivers.

Home

A specific location in the machine’s work envelope that is used to make sure the computer and the actual machine both agree on the tool position.

ini file

A text file that contains most of the information that configures EMC for a particular machine

Instance

One can have an instance of a class or a particular object. The instance is the actual object created at runtime. In programmer jargon, the Lassie object is an instance of the Dog class.

Joint Coordinates

These specify the angles between the individual joints of the machine. See also Kinematics

Jog

Manually moving an axis of a machine. Jogging either moves the axis a fixed amount for each key-press, or moves the axis at a constant speed as long as you hold down the key.

kernel-space

See real-time.

Kinematics

The position relationship between world coordinates and joint coordinates of a machine. There are two types of kinematics. Forward kinematics is used to calculate world coordinates from joint coordinates. Inverse kinematics is used for exactly opposite purpose.Note that kinematics does not take into account, the forces, moments etc. on the machine. It is for positioning only.

Lead-screw

An screw that is rotated by a motor to move a table or other part of a machine. Lead-screws are usually either ball-screws or acme screws, although conventional triangular threaded screws may be used where accuracy and long life are not as important as low cost.

Machine units

The linear and angular units used for machine configuration. These units are used in the inifile. HAL pins and parameters are also generally in machine units.

MDI

Manual Data Input. This is a mode of operation where the controller executes single lines of G-code as they are typed by the operator.

NIST

National Institute of Standards and Technology. An agency of the Department of Commerce in the United States.

Offsets

Part Program

A description of a part, in a language that the controller can understand. For EMC, that language is RS-274/NGC, commonly known as G-code.

Program Units

The linear and angular units used for part programs.

Python

General-purpose, very high-level programming language. Used in EMC2 for the Axis GUI, the Stepconf configuration tool, and several G-code programming scripts.

Rapid

Fast, possibly less precise motion of the tool, commonly used to move between cuts. If the tool meets the material during a rapid, it is probably a bad thing!

Real-time

Software that is intended to meet very strict timing deadlines. Under Linux, in order to meet these requirements it is necessary to install RTAI or RTLINUX and build the software to run in those special environments. For this reason real-time software runs in kernel-space.

RTAI

Real Time Application Interface, see https://www.rtai.org/, one of two real-time extensions for Linux that EMC can use to achieve real-time performance.

RTLINUX

See http://www.rtlinux.org, one of two real-time extensions for Linux that EMC can use to achieve real-time performance.

RTAPI

A portable interface to real-time operating systems including RTAI and RTLINUX

RS-274/NGC

The formal name for the language used by EMC part programs.

Servo Motor

A special kind of motor that uses error-sensing feedback to correct the position of an actuator.

Servo Loop

A control loop used to control position or velocity of an motor equipped with a feedback device.

Signed Integer

A whole number that can have a positive or negitive sign. (-12) In HAL it is known as s32.

Spindle

On a mill or drill, the spindle holds the cutting tool. On a lathe, the spindle holds the workpiece.

Spindle Speed Override

A manual, operator controlled change in the rate at which the tool rotates while cutting. Often used to allow the operator to adjust for chatter caused by the cutter’s teeth. Spindle Speed Override assume that the EMC2 software has been configured to control spindle speed.

Stepconf

An EMC2 configuration wizard. It is able to handle many step-and-direction motion command based machines. Writes a full configuration after the user answers a few questions about the computer and machine to be run with.

Stepper Motor

A type of motor that turns in fixed steps. By counting steps, it is possible to determine how far the motor has turned. If the load exceeds the torque capability of the motor, it will skip one or more steps, causing position errors.

TASK

The module within EMC that coordinates the overall execution and interprets the part program.

Tcl/Tk

A scripting language and graphical widget toolkit with which several of the EMC’s GUI’s and selection wizards were written.

Traverse Move

A move in a straight line from the start point to the end point.

Units

See "Machine Units", "Display Units", or "Program Units".

Unsigned Integer

A whole number that has no sign. (123) In HAL it is known as u32.

World Coordinates

This is the absolute frame of reference. It gives coordinates in terms of a fixed reference frame that is attached to some point (generally the base) of the machine tool.

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GNU Free Documentation License

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ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

Copyright (c) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. A copy of the license is included in the section entitled "GNU Free Documentation License".

If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being LIST"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.