Octoprint GRBL Error 3

[scottmil]

I'm using the Octoprint GRBL plugin with my laser engraver running GRBL 1.1f. The engraver connects just fine, but when I begin to print the engraving stops after the firmware reports an error 3 (unknown gcode $ command) and the engraver crashes into an end-stop. However, by examining my the serial.log, I cannot see any incorrect $ gcodes:

https://gist.github.com/scottmil/ea4a7d47d5a8c1d42ac7d604a5970fe7

I'm running Octoprint 1.3.4 on a Raspberry Pi with the latest version of Raspian Jessie.

BTW: Thanks for creating this plugin. I hope I can get it to work.

[scottmil]

I increased the Octoprint temperature interval to 10 seconds, and now my engraver is able to respond with "ok" after each temperature M105 (translated to ?$G). I no longer see Error 3 in my serial log.

However, when I try to print/engrave, it appears (from serial.log) that the engraver buffer cannot handle the rapid stream of Octoprint commands, and eventually the GRBL firmware issues an Error 9. Is there a way to force the GRBL to respond with "ok" before Octoprint issues the next command, maybe by using xon/xoff?

log file: https://gist.github.com/scottmil/76c6e1af13eb53e984bc9aab5f950c7d