Tinyg Communications Programming

Rob Giseburt edited this page Nov 21, 2017 · 74 revisions

If you are on this page we can assume you want to write a program that talks to TinyG to send Gcode files, and possibly also to read and set configuration variables, report machine status, control jogging and homing, and other functions. It is intended for GUI developers and other low-level access.

We strongly recommend using the node-g2core-api nodeJS module, which already handles the communications. This page is useful if for some reason you need to write your own communications handler, or just want to know how it works._

If you are just looking for an off-the-shelf way to drive TinyG please see these other links:

Introduction

This page discusses issues and approaches to writing a good programatic interface to TinyG/g2core. It may also be useful to review the introduction to the tinyg code base. This page on flow control and footers also has some notes that were used in developing the system communications.

If you are writing a programmatic interface we highly recommend that you use the JSON syntax and avoid the command line (plain text) form. Text mode is really just a convenience for driving the interface from a command line for debugging and system discovery. All examples here are provide in JSON form.

If you want to skip the background info you can skip right to the linemode protocol

Vocabulary

By board we mean and TinyG or g2core flavored controller board.

A host is any computer that talks to a board - typically an OSX, Linux, or Windows laptop or desktop computer communicating over USB, or a microhost like a Beaglebone, Raspberry Pi, or NextThingCo CHIP. Microhosts usually talk to a board over a direct serial UART - although they may alternately communicate over USB.

A command is a single line of text sent from the host to the board. A command can be either data or control. Gcode lines are always data commands. Generally, unless a line is identified as a control, it's considered data. JSON commands are always controls. So are single-character commands such as ! feedhold, % queue flush, ~ resume

Multiplexed Channels: Even though there is only a single physical host-to-board connection (USB or serial) the communications channel distinguishes between control and data lines and keeps separate logical queues for each. Controls are always executed before data - so they "jump the queue".

Theory of Operation

TinyG communicates over a single USB serial channel terminated by an FTDI chip (USB serial emulation). The default baud rate is 115,200 baud, but can be set to values between 9600 and 230,400 using the {"baud":N} command. See Configuring TinyG.

g2core is configured similarly, but uses a native USB channel that will transmit at 12 Mbps or 480Mbps, depending on the exact board and host used. It can alternately be connected to a using a 4-wire serial port (rx/tx/rts/cts).

The board has at least a 1000 byte serial buffer that can buffer up to 8 typical lines of ASCII text. A "command" is a line of ASCII text ending with a CR or LF; or one or the other depending on the {"ic":N} setting. The "controller" pulls serial lines from the buffer and passes them to the correct parser for that type of command. There are 4 general classes of commands that can be pulled from the serial buffer:

Command Type Example(s)
Gcode blocks (data) g0x10, m7, g17
Configuration (control) {"xvm":16000} (set X maximum velocity to 16000), {"1mi":8} (set motor 1 microsteps to 8)
Action (control) {"defa":1} (reset config values to default), {"sr":null} (request a status report)
Front-Panel (control) ! (feedhold), ~ (cycle start)

Commands from the serial buffer are handled as:

Gcode Blocks

When a Gcode block is encountered it is passed to the Gcode parser. Depending on the gcode command it is either executed immediately or queued to the motion planner. All motion commands, dwells and most M commands are queued to the planner - i.e. "synchronized with motion". Examples of commands that are executed immediately are G20 and G21 - which set inches and millimeter units, respectively. These are executed immediately as the next gcode block that arrives needs to be interpreted in the correct units.

In machinist-speak Gcode commands are "cycle" commands. This means that Gcode commands execute after a cycle start. (In fact, the firmware performs an automatic cycle start when it receives a Gcode command if it's not already in a machining cycle). Gcode cycle commands manipulate the internal system state model. When the Gcode "file" is done it is supposed to be terminated with a Program End (M2 or M30), indicating that the machining cycle is over.

This in-cycle / out-of-cycle is an important distinction because (notwithstanding some exceptions) configuration and actions should only occur out-of-cycle or errors and undesirable behaviors can creep in. This is discussed later.

Another way to look at Gcode commands is that the in-cycle commands manipulate the "dynamic model" of the machine. The dynamic model is typically what is returned from status reports. In RESTful terms, the dynamic model is a stateful Resource.

Note that Gcode blocks are the exception to JSON mode. Gcode can be sent either wrapped in JSON or as native ASCII. Both of the following are acceptable forms: g0x10 or {"gc":"g0x10"}.

Configuration Commands

Configuration commands are all the motor settings, axis settings, PWM settings, system settings, and other things that should be set up before you enter a cycle. This is the "static model" for the machine, and is represented by a series of stateful Resources (e.g. {"x":null}, {"1":null}, {"sys":null} )

Reading a configuration command (e.g. {"xvm":null} or {"x":null} ) is a no-brainer as it's just a retrieval.

Writing a config value (e.g. {"xvm":16000} ) is more complicated. Writing usually means that the new value is persisted to non-volatile memory (aka NVM, EEPROM). On the Xmega, at least, this persistence operation is painful in that it must disable all interrupts while the NVM write occurs. This means 2 things:

  1. You cannot do a configuration write during a machining cycle as the steppers will stop
  2. There can be no serial activity the duration of the write (something < 30 ms)

The simplest way to deal with this is to (1) don't issue config commands during a cycle, and (2) always run configuration commands synchronously. In other words, always wait until you receive the response from a command before sending the next one. Do not just blast them down to the serial buffer as you would a Gcode file.

Action Commands

There are a small number of commands that look like configs but actually perform actions or return 'reports". These are:

Command Description
{"sr":null} Request a status report
{"defa":1} Reset configuration to default
{"test":N} Run self test N
{"boot":1} Enter boot loader
{"help":null} Request help screen

Obviously these commands should not be run during a machining cycle.

Front-Panel Commands

These commands are the types of things that you might find on the front panel of a CNC machine. They effect the dynamic model (i.e. are not config or action commands), and are not found in the gcode "file". Currently there are only two front panel commands, but at some point we expect there will be more.

Command Description
! Feedhold (pause)
~ Cycle start (resume)

Driving the Board from a Host Computer

Now that we've discussed how commands work let's talk about how to feed commands to TinyG for optimal program execution.

We recommend using Line Mode protocol for host-to-board communications. In line mode the board works on complete command lines, instead of characters. The host sends a few command lines to prime the board's serial receive queue, then the host sends a new line each time it receives a response from a processed line. That's it. So if you are building a sender that's all you need to do.

We recommend using the node-g2core-api nodeJS module, which already handles line mode communications. Or just use it as a worked example if you are writing your own.

TinyG and g2core also support character mode (byte streaming) which is deprecated and may be removed at a later time.

Linemode Protocol

Linemode protocol was designed to help prevent the serial buffer from either filling completely (preventing time-critical commands from getting through) while keeping the serial buffer full enough in order to prevent degradation to motion quality due to the motion commands not making it to the machine in a timely manner. Lines are processed in the order the command lines were received, but any control commands take precedence over data commands and are processed first.

Linemode allocates 8 "line slots" - the board can hold 8 lines of text before declaring that it has no more room in the receive buffer. It's been determined that 8 is a good number, it allows 4 slots to be always filled with data (gcode), while leaving slots open for controls. This keeps the Gcode flowing and allows responsive controls.

The protocol is simple - "blast" 4 command lines to the board without waiting for responses. From that point on send a single line for every {r:...} response received. Every command will return one and only one response. The exceptions are single character commands, such as !, ~, or ENQ, which do not consume line buffers and do not generate responses.

In implementation it's actually rather simple:

  1. Prepare or start reading the list of data lines to send to the g2core. We'll call this list line_queue.
  2. Set lines_to_send to 4.
  • 4 has been determined to be a good starting point. This is subject to tuning, and might be adjusted based on your results.
  1. Send the first lines_to_send lines of line_queue to the g2core, decrementing lines_to_send by one for each line sent.
  • If you need to read more lines into line_queue, do so as soon as lines_to_send is zero.
  • If line_queue is being filled by dynamically generated commands then you can send up to lines_to_send lines immedately.
  • Don't forget to decrement lines_to_send for each line sent! And don't send more than lines_to_send lines!
  1. When a {r:...} response comes back from the g2core, add one to lines_to_send.
  • It's vital that when any {r:...} comes back that lines_to_send. If one is lost or ignored then the system will get out of sync and sending will stall.
  1. Loop back to 3. (Better yet, have 3 and 4 each loop in their own thread or context.)

Notes:

  • Steps 3 and 4 are best to run in their own threads or context. (In node we have each in event handlers, for example.) If that's not practical, it's vital that when a {r:...} comes in that lines_to_send is incremented and that lines can be sent as quickly after that as possible.
  • It is possible (and common) to get two or more {r:...} responses before you send another line. This is why it's vital to keep track of lines_to_send.
  • Note that only data (gcode) lines go into line_queue! For configuration JSON or single-line commands, they are sent immediately.
    • It's important to maintain lines_to_send even when sending past the line_queue.
      • Single-character commands will not generate a {r:...} response (they may generate other output, however), so there's nothing to do (see following notes).
      • JSON commands ({ being the first character on the line) will have a {r:...} response, so when you send one of those past the queue you should still subtract one from lines_to_send, or lines_to_send will get out of sync and the sender will eventually stall waiting for responses. This is the only case where lines_to_send may go negative.
    • Note that control commands, like dta commands, must start at the beginning of a line, so you should always send whole lines. IOW, don't interrupt a line being sent from the line_queue to send a JSON command or feedhold !.
  • All communications to the g2core must go through this protocol. It's not acceptable to occasionally send a JSON command or gcode line directly past this protocol, or lines_to_send will get out of sync and the sender will eventually stall waiting for responses.

Resyncing

If the host somehow loses responses the line count could potentially get out of sync. We have only seen this in two cases - one where the host's internal program flow was dropping responses due to inter-process communications and mutual exclusion problems, and once when there was a bug in the g2core USB stack that prevented occasional USB packets from getting through (this was fixed). In any case, a belt and suspenders approach is for the host to time out responses, and examine the last received buffer count to re-sync.

The number of line buffers available is returned as the third (last) number in the {r:...} response footer, and can also be queried by sending an {rx:n} command. You will never see more than 7, because the command being processed is one of the 8. However, in normal operation the host will not use the available lines count, as this is only necessary to handle some exceptional cases.

The buffer count can be tested this way, but realize that the only truly accurate count will be if the job has stalled. Otherwise the asynchronous nature of the communications and processing may be off by one. However, if you see there are only 0 or 1 buffers available it may be a good idea to back off sending until the buffer count comes back up to 4 or 5 in subsequent response footers.

Backgrounder: The "Bucket Brigade" Problem

Bucket Brigade

With a single channel of communication, either UART or USB, we have two single-file lines of bytes: one going to the g2core, and one coming from the g2core.

In both cases, there are often several "players" involved between the two endpoints of the channel. For example, for USB, there is:

  • Your UI application →
  • The language's serial buffers →
  • The OS serial USB driver buffers →
  • The OS USB buffers →
  • The g2core USB peripheral buffers →
  • The g2core serial buffers, then, finally →
  • The g2core line parser.

This amounts to a "bucket brigade," where once you pass the line into the serial system, it has to pass through several "hidden" layers (there may be more or less than listed above) before it gets to the g2core, and then there's a buffer before it can be parsed and handled.

This is generally fine, as long as you don't need to stop the flow of commands. For example, to execute a feed-hold you send a ! character at the beginning of a new line. It has to get through all of those buffers before it is even seen by the g2core. So, if there are too many lines in the buffer before the g2core, the feedhold may take a noticeable (and unacceptable) amount of time to go into effect.

To complicate matters, the g2core cannot blindly read lines, but only reads lines as there is room for the parsed results. This generally means that as a line is read and parsed, room is made in the internal serial buffer. If the motion buffer is full, however, there is no room for new moves to be stored, so g2core will stop reading and parsing data lines until room is made in the motion buffer. It will continue to read control lines in any case.

To help with this the g2core serial system will "pre-parse" the incoming serial buffer, even when data lines are no longer being read (due to the motion buffer being full, a feedhold in place, etc), and look for lines that begin with "{" or one of the single-character commands (!, ~, %, ^D, ^X, etc.). If it locates one of these lines, it will mark it as a control line and the next control-line-read will see it and execute it immediately.

So, as long as the serial buffer is not completely filled with data lines, there is at least room for a single-line command and generally room for a JSON command as well.

There are other issues that are dealt with in other ways, such as if the serial buffer does fill completely (even if very briefly) we need to ensure that we don't lose data. This is mostly a concern with UART, and in that case we require some form of flow control. Currently we only support hardware flow-control, using the additional RTS/CTS lines. Linemode is not a replacement for flow-control, and will not prevent lost data due to buffer overflow on it's own.

One further note: Due to the way g2core plans motion in real-time, if the gcode commands request moves of very brief duration, and the serial buffer isn't kept full enough of moves, then there will be a noticeable degradation in velocity and in some configurations the machine will exhibit a noticeable "stutter" as it executes each move on it's own or in small groups.

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