This documentation is no longer maintained. For documentation of the current version of emc2, please see http://www.linuxcnc.org/docview/html

1 General Reference Information

1.1 Notation

1.1.1 Typographical Conventions

Command line examples are presented in bold typewriter font. Responses from the computer will be in typewriter font. As of early 2006, there are no longer commands that require root privileges, so all examples will be preceded by the normal user prompt, $. Text inside square brackets [like-this] is optional. Text inside angle brackets <like-this> represents a field that can take on different values, and the adjacent paragraph will explain the appropriate values. Text items separated by a vertical bar means that one or the other, but not both, should be present. All command line examples assume that you are in the emc2/ directory, and you configured/compiled emc2 for the run-in-place scenario. Paths will be shown accordingly when needed.

1.1.2 Names

All HAL entities are accessed and manipulated by their names, so documenting the names of pins, signals, parameters, etc, is very important. HAL names are a maximum of 41 characters long (as defined by HAL_NAME_LEN in hal.h). Many names will be presented in a general form, with text inside angle brackets <like-this> representing fields that can take on different values.

When pins, signals, or parameters are described for the first time, their names will be preceeded by their type in (small caps) and followed by a brief description. A typical pin definition will look something like these examples:

(bit) parport.<portnum>.pin-<pinnum>-in -- The HAL pin associated with the physical input pin <pinnum> on the 25 pin D-shell connector.
(float) pid.<loopnum>.output -- The output of the PID loop.

At times, a shortened version of a name may be used - for example the second pin above might be referred to simply as .output when it can be done without causing confusion.

1.2 General Naming Conventions

Consistent naming conventions would make HAL much easier to use. For example, if every encoder driver provided the same set of pins and named them the same way it would be easy to change from one type of encoder driver to another. Unfortunately, like many open-source projects, HAL is a combination of things that were designed, and things that simply evolved. As a result, there are many inconsistencies. This section attempts to address that problem by defining some conventions, but it will probably be a while before all the modules are converted to follow them.

Halcmd and other low-level HAL utilities treat HAL names as single entities, with no internal structure. However, most modules do have some implicit structure. For example, a board provides several functional blocks, each block might have several channels, and each channel has one or more pins. This results in a structure that resembles a directory tree. Even though halcmd doesn't recognize the tree structure, proper choice of naming conventions will let it group related items together (since it sorts the names). In addition, higher level tools can be designed to recognize such structure, if the names provide the neccessary information. To do that, all HAL modules should follow these rules:

Dots (“.”) separate levels of the heirarchy. This is analogous to the slash (“/”) in a filename.
Hypens (“-”) separate words or fields in the same level of the heirarchy.
HAL modules should not use underscores or “MixedCase”. ¹
Use only lowercase letters and numbers in names.

1.3 Hardware Driver Naming Conventions²

1.3.1 Pin/Parameter names

Hardware drivers should use five fields (on three levels) to make up a pin or parameter name, as follows:

<device-name>.<device-num>.<io-type>.<chan-num>.<specific-name>

The individual fields are:

<device-name>: The device that the driver is intended to work with. This is most often an interface board of some type, but there are other possibilities.
<device-num>: It is possible to install more than one servo board, parallel port, or other hardware device in a computer. The device number identifies a specific device. Device numbers start at 0 and increment.³
<io-type>: Most devices provide more than one type of I/O. Even the simple parallel port has both digital inputs and digital outputs. More complex boards can have digital inputs and outputs, encoder counters, pwm or step pulse generators, analog-to-digital converters, digital-to-analog converters, or other unique capabilities. The I/O type is used to identify the kind of I/O that a pin or parameter is associated with. Ideally, drivers that implement the same I/O type, even if for very different devices, should provide a consistent set of pins and parameters and identical behavior. For example, all digital inputs should behave the same when seen from inside the HAL, regardless of the device.
<chan-num>: Virtually every I/O device has multiple channels, and the channel number identifies one of them. Like device numbers, channel numbers start at zero and increment.⁴ If more than one device is installed, the channel numbers on additional devices start over at zero. If it is possible to have a channel number greater than 9, then channel numbers should be two digits, with a leading zero on numbers less than 10 to preserve sort ordering. Some modules have pins and/or parameters that affect more than one channel. For example a PWM generator might have four channels with four independent “duty-cycle” inputs, but one “frequency” parameter that controls all four channels (due to hardware limitations). The frequency parameter should use “0-3” as the channel number.
<specific-name>: An individual I/O channel might have just a single HAL pin associated with it, but most have more than one. For example, a digital input has two pins, one is the state of the physical pin, the other is the same thing inverted. That allows the configurator to choose between active high and active low inputs. For most io-types, there is a standard set of pins and parameters, (referred to as the “canonical interface”) that the driver should implement. The canonical interfaces are described in chapter [.].

1.3.1.1 Examples

motenc.0.encoder.2.position: -- the position output of the third encoder channel on the first Motenc board.
stg.0.din.03.in: -- the state of the fourth digital input on the first Servo-to-Go board.
ppmc.0.pwm.00-03.frequency: -- the carrier frequency used for PWM channels 0 through 3.

1.3.2 Function Names

Hardware drivers usually only have two kinds of HAL functions, ones that read the hardware and update HAL pins, and ones that write to the hardware using data from HAL pins. They should be named as follows:

<device-name>-<device-num>[.<io-type>[-<chan-num-range>]].read|write

<device-name>: The same as used for pins and parameters.
<device-num>: The specific device that the function will access.
<io-type>: Optional. A function may access all of the I/O on a board, or it may access only a certain type. For example, there may be independent functions for reading encoder counters and reading digital I/O. If such independent functions exist, the <io-type> field identifies the type of I/O they access. If a single function reads all I/O provided by the board, <io-type> is not used.⁵
<chan-num-range>: Optional. Used only if the <io-type> I/O is broken into groups and accessed by different functions.
read|write: Indicates whether the function reads the hardware or writes to it.

1.3.2.1 Examples

motenc.0.encoder.read: -- reads all encoders on the first motenc board
generic8255.0.din.09-15.read: -- reads the second 8 bit port on the first generic 8255 based digital I/O board
ppmc.0.write: -- writes all outputs (step generators, pwm, DACs, and digital) on the first ppmc board

2 Canonical Device Interfaces⁶

The following sections show the pins, parameters, and functions that are supplied by “canonical devices”. All HAL device drivers should supply the same pins and parameters, and implement the same behavior.

Note that the only the <io-type> and <specific-name> fields are defined for a canonical device. The <device-name>, <device-num>, and <chan-num> fields are set based on the characteristics of the real device.

2.1 Digital Input

The canonical digital input (I/O type field: digin) is quite simple.

2.1.1 Pins

(bit) in -- State of the hardware input.
(bit) in-not -- Inverted state of the input.

2.1.2 Parameters

None

2.1.3 Functions

(funct) read -- Read hardware and set in and in-not HAL pins.

2.2 Digital Output

The canonical digital output (I/O type field: digout) is also very simple.

2.2.1 Pins

(bit) out -- Value to be written (possibly inverted) to the hardware output.

2.2.2 Parameters

(bit) invert -- If TRUE, out is inverted before writing to the hardware.

2.2.3 Functions

(funct) write -- Read out and invert, and set hardware output accordingly.

2.3 Analog Input

The canonical analog input (I/O type: adcin). This is expected to be used for analog to digital converters, which convert e.g. voltage to a continuous range of values.

2.3.1 Pins

(float) value -- The hardware reading, scaled according to the scale and offset parameters. Value = ((input reading, in hardware-dependent units) * scale) - offset

2.3.2 Parameters

(float) scale -- The input voltage (or current) will be multiplied by scale before being output to value.
(float) offset -- This will be subtracted from the hardware input voltage (or current) after the scale multiplier has been applied.
(float) bit_weight -- The value of one least significant bit (LSB). This is effectively the granularity of the input reading.
(float) hw_offset -- The value present on the input when 0 volts is applied to the input pin(s).

2.3.3 Functions

(funct) read -- Read the values of this analog input channel. This may be used for individual channel reads, or it may cause all channels to be read

2.4 Analog Output

The canonical analog output (I/O Type: adcout). This is intended for any kind of hardware that can output a more-or-less continuous range of values. Examples are digital to analog converters or PWM generators.

2.4.1 Pins

(float) value -- The value to be written. The actual value output to the hardware will depend on the scale and offset parameters.
(bit) enable -- If false, then output 0 to the hardware, regardless of the value pin.

2.4.2 Parameters

(float) offset -- This will be added to the value before the hardware is updated
(float) scale -- This should be set so that an input of 1 on the value pin will cause 1V
(float) high_limit (optional) -- When calculating the value to output to the hardware, if value + offset is greater than high_limit, then high_limit will be used instead.
(float) low_limit (optional) -- When calculating the value to output to the hardware, if value + offset is less than low_limit, then low_limit will be used instead.
(float) bit_weight (optional) -- The value of one least significant bit (LSB), in volts (or mA, for current outputs)
(float) hw_offset (optional) -- The actual voltage (or current) that will be output if 0 is written to the hardware.

2.4.3 Functions

(funct) write -- This causes the calculated value to be output to the hardware. If enable is false, then the output will be 0, regardles of value, scale, and offset. The meaning of “0” is dependent on the hardware. For example, a bipolar 12-bit A/D may need to write 0x1FF (mid scale) to the D/A get 0 volts from the hardware pin. If enable is true, read scale, offset and value and output to the adc (scale * value) + offset. If enable is false, then output 0.

2.5 Encoder

The canonical encoder interface (I/O type field: encoder ) provides the functionality needed for homing to an index pulse and doing spindle synchronization, as well as basic position and/or velocity control. This interface should be implementable regardless of the actual underlying hardware, although some hardware will provide “better” results. (For example, capture the index position to +/- 1 count while moving faster, or have less jitter on the velocity pin.)

2.5.1 Pins

(s32) count -- Encoder value in counts.
(float) position -- Encoder value in position units (see parameter “scale”).
(float) velocity -- Velocity in position units per second.
(bit) reset -- When True, force counter to zero.
(bit) index-enable -- (bidirectional) When True, reset to zero on next index pulse, and set pin False.

The “index-enable” pin is bi-directional, and might require a little more explanation. If “index-enable” is False, the index channel of the encoder will be ignored, and the counter will count normally. The encoder driver will never set “index-enable” True. However, some other component may do so. If “index-enable” is True, then when the next index pulse arrives, the encoder counter will be reset to zero, and the driver will set “index-enable” False. That will let the other component know that an index pulse arrived. This is a form of handshaking - the other component sets “index-enable” True to request a index pulse reset, and the driver sets it False when the request has been satisfied.

2.5.2 Parameters

(float) scale -- The scale factor used to convert counts to position units. It is in “counts per position unit”. For example, if you have a 512 count per turn encoder on a 5 turn per inch screw, the scale should be 512*5 = 2560 counts per inch, which will result in “position” in inches and “velocity” in inches per second.
(float) max-index-vel -- (optional) The maximum velocity (in position units per second) at which the encoder can reset on an index pulse with +/- 1 count accuracy. This is an output from the encoder driver, and is intended to tell the user something about the hardware capabilities. Some hardware can reset the counter at the exact moment the index pulse arrives. Other hardware can only tell that an index pulse arrived sometime since the last time the read function was called. For the latter, +/- 1 count accuracy can only be achieved if the encoder advances by 1 count or less between calls to the read function.
(float) velocity-resolution -- (optional) The resolution of the velocity output, in position units per second. This is an output from the encoder driver, and is intended to tell the user something about the hardware capabilities. The simplest implementation of the velocity output is the change in postion from one call of the read function to the next, divided by the time between calls. This yields a rather coarse velocity signal that jitters back and forth between widely spaced possible values (quantization error). However, some hardware captures both the counts and the exact time when a count occurres (possibly with a very high resolution clock). That data allows the driver to calculate velocity with finer resolution and less jitter.

2.5.3 Functions

There is only one function, to read the encoder(s).

(funct) read -- Capture counts, update position and velocity.

Footnotes

1 Underscores have all been removed, but there are still a few instances of mixed case, for example “pid.0.Pgain” instead of “pid.0.p-gain”. back

2 Most drivers do not follow these conventions as of version 2.0. This chapter is really a guide for future development. back

3 Some devices use jumpers or other hardware to attach a specific ID to each board. Ideally, the driver provides a way for the user to specifically say “device-num 0 is the board with ID XXX”, and the device numbers always start at 0. However at present some drivers use the board ID directly as the device number. That means it is possible to have a device number 2, without a device 0. This is a bug and will be fixed in version 2.1. back

4 One glaring exception to the “channel numbers start at zero” rule is the parallel port. Its HAL pins are numbered with the corresponding pin number on the DB-25 connector. This is convenient for wiring, but inconsistent with other drivers. There is some debate over whether this is a bug or a feature. back

5 Note to driver programmers: do NOT implement separate functions for different I/O types unless they are interruptable and can work in independent threads. If interrupting an encoder read, reading digital inputs, and then resuming the encoder read will cause problems, then implement a single function that does everything. back

6 As of version 2.0, most of the HAL drivers don't quite match up to the canonical interfaces defined here. In version 2.1, the drivers will be changed to match these specs. back

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