JSON Active Comments

This page describes using Gcode comments to carry JSON commands such that they can extend existing Gcode functions or insert control functions that execute synchronously with the Gcode.

##Gcode Comments Gcode comments possess a number of features that extend their capabilities beyond classic Gcode. Classic Gcode only defines parentheses comments (...) and message comments (msg...). Additionally, a slash character / in the first character is a block delete, which can be considered a type of comment. G2 handles comments as so:

• Classic Gcode comments that start with a '(' and end with a ')', known as "inline comments", will be removed, UNLESS they are an active comment
• Gcode message active comments are comments with the letters msg (case insensitive) immediately following the open parenthesis: (Msg....)
  • The string enclosed by the trailing g and the closing paren will be returned in the next status report as a "msg":"...." tag
  • If there is a space or anything else between ( and the m then it will be treated as a normal comment
  • Only one message active comment is allowed per Gcode block
• JSON Active Comments are comments with JSON immediately inside the parentheses: ({...})
  • Active comments carry content that GCode can't express, but is intrinsically part of the command on that line
  • If there's a space or anything else between the ( and the { then it will be treated as a normal comment
• Multiple inline comments (...) are allowed, and gcode and active comments in between will be retained
• Multiple active comments are allowed, and will be combined and treated as if they were separated by commas
  • Ex: M100 ({he1st:200}) ({he2st:210}) will be treated the same as M100 ({he1st:200, he2st:210})
  • Internally msg comments are converted to JSON comments, so these may also be mixed with JSON active comments
• Comments that start with a semicolon ';' end the line -- everything including and after the ';' will be ignored. SOMe Gcode generators use semicolons as comments.

Valid comment cases	Notes:
G0X10	command only - no comment
G0X10 (comment text)	command with comment
G0X10 (comment text	acceptable, but a warning may be issued for the comment not being terminated
G0X10 ;comment text	comment delimited by semicolon
(MSGSend this string)	will report back as "msg":"Send this string"
(comment text)	there is no command on this line
G0 (traverse) X10 (to X ten) Y12 (and Y twelve)	Command G0X10Y12 with multiple inline comments
M100 (set heater temp to 210:) ({he1st:210})	Command with inline comment and active comment

Additional considerations:

The % character is commonly used at the beginning and end of a gcode file to delimit the file.

We don't need any special handling in these cases. M2 and M30 handle the "end-of-job" case, and should be used for that purpose. We do NOT plan on supporting running two Gcode files concatenated. Or, at least, we don't plan on adding any special handling for that case. The % character is used by some gcode generators (InkScape, for example) as a start-comment character.

Using % as a "buffer flush" doesn't make any sense without a ! "feed hold" proceeding it.