GCode M117 Display Message Command

Hello,
I'm a long time observer of 3D printing and just recently found the time and money to get into it. I'm also getting into the firmware and trying to write some post processors for gcode.
I was wondering why the firmwares uses the M command 117 to display a message on the LCD. The NIST 274/NGC standard states that comments with "MSG" are to be displayed (3.3.4 Comments and Messages). Using M117 violates the standard pretty heavily, by breaking the format of a line (3.3 Format of a Line). Command M118 also has this issue.
I don't mean to come off as righteous, specially being new to the community, but it seems like a bad idea to violate the standard. Is there a reason this was done this way, am I missing something? As I try to write libraries to work with gcode I'd like to be able to maintain as much cross compatibility as possible when it comes to interpreting the lines.

If this is not the right place to discuss the firmware details I would appreciate being pointed in the right direction.

Thanks,
oz

The definition of G- and M-codes for 3D printing has diverged a long way from the NIST standard. In some cases this was necessary, because 3D printers have many features that CNC machines do not. In other cases, firmware developers blatantly disregarded the existing standard, for example by defining G-codes to do something when they already had a completely different meaning in NIST. This is a pity as makes it harder to write a single firmware that supports both 3D printers and CNC machines.

The NIST standard requires comments to be placed in round brackets; but 3D printing firmwares generally assume that anything after a semicolon to the end of the line is a comment. That may be one reason why MSG comments are not handled. You are right, M117 is rather limited and breaks the line style. That's why we defined M291 in RepRapFirmware, see [reprap.org].

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].