Protocol Wire Format

Wire format

What strings Python sends the controller, how it knows when it's received a full response, how it detects the driver version.

Basic format

sent:      <command>[:<param>...]\n
received:  <code>:<message>

Code 0 is success, 1 is failure.

Two easy things to trip on:

• The command sent has to end in a newline; the driver's READ comm_file(cmd) reads line by line.
• The response has no terminator character. WRITE comm_file (resp) in mappdk_server.kl doesn't send a CR; the only boundary is the TCP packet boundary. Upstream therefore only calls recv() once, so a response split across packets reads as truncated. Here the protocol layer tells the transport layer whether it's "received enough yet"; if not, it keeps reading, see transport.MappdkTransport._recv.

When a connection is established, the driver sends a 0:success first (in OPEN_COMM, mappdk_comm.kl), which has to be read and discarded, or it gets mistaken for the response to the next command.

Motion command field widths

movej:VVVV:AAAA:CCC:M:N:<value1>:<value2>...
       │    │    │  │ └ axis count, 1 digit
       │    │    │  └ 0=joint interpolation 1=linear interpolation
       │    │    └ CNT, 3 digits
       │    └ acceleration, 4 digits
       └ velocity, 4 digits

Each value is a sign plus a 13-character fixed-point number, 14 characters total, e.g. +000090.000000. The 13-character width is 6 integer digits + decimal point + 6 fraction digits, not the integer digit count; easy to misread.

Version detection

FanucRobot.connect() sends ver. The upstream driver replies wrong-command, at which point robot.extended is set to False, and calling an extended method afterward states the reason directly; the RDO number limit switches accordingly too.

A latent packet-boundary issue

transport.py's original default is_complete only checked "is there a colon", so a message long enough to happen to land right on a TCP packet boundary could be judged complete too early, leaking leftover bytes into the next response. _recv now does a short-timeout extra read after judging a response complete, to confirm there's no follow-up data; this costs essentially no time in the normal case.

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Last updated: 2026-08-31