Weird issue with Start-GCODE  

Honorable Member

Maybe someone has an idea of why this is happening:

I was contacted by someone who downloaded one of my GCODEs and noticed some underextrusion. He was able to fix it by commenting out a line in the gcode:

M907 E430

He contacted me and we went back and forth on it, and we discovered that on my system (I'm using RC3) there's the following in the Start G-code section for the "Original Prusa i3 Mk3S & Mk3S+" printer profile:

; Don't change E values below. Excessive value can damage the printer.
{if print_settings_id=~/.*(DETAIL @MK3|QUALITY @MK3).*/}M907 E430 ; set extruder motor current{endif}
{if print_settings_id=~/.*(SPEED @MK3|DRAFT @MK3).*/}M907 E538 ; set extruder motor current{endif}

which obviously adds the M907 to the GCODE.

Now here's where I'm lost. In his install, he doesn't have that block of code in the start gcode of the stock profile. He said he checked both RC2 and RC3.

For the fun of it, I just did a clean re-install of RC3 on a different computer, and sure enough the M907 stuff is in the custom g-code.

So why wouldn't it be in his, and why would the M907 E430 lead to underextrusion for him while I've never seen it on my printer?

Posted : 11/01/2021 2:31 pm
Illustrious Member

The addition of those lines in the default Prusa PrusaSlicer defaults is relatively new. I believe they came in after the release of v2.2. A couple of possibilities then:

  • If you've changed anything in one of the Prusa default printer profiles and saved it off as a custom profile, updated start gcode won't be applied to the custom profile. 
  • If your buddy simply hasn't updated the profiles in some time, he could have the older version.
  • Those conditions will only match if the strings tested for match. If he is using a print settings profile that does NOT have "DETAIL @MK3", "QUALITY @MK3", "SPEED @MK3", or "DRAFT @MK3" in the print settings ID, neither of those lines will match and no gcode will be inserted.

If your buddy is using customized profiles, or a non-Prusa printer, that would certainly explain it.

I'm not a fan of the M907 settings and don't use them in my custom gcode. It shouldn't cause print failures to have it present, but be aware that the setting will remain in use until you either reset it explicitly in gcode or reset the printer.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 11/01/2021 3:23 pm
Famed Member

Prusa recently added those in to the profiles.  Basic profiles get bundled with a release but they get updated from the prusa servers periodically.  There have been scattered reports of the updates not working for some people.  Possibly down to network restrictions/security etc.  No one knows as yet.

The Gcode added is to tune the stepper motor currents.  I'm sure some printers might have problems with that and so cause underextrusion.

Personally I've rebuilt my extruder as a geared one so I stripped those lines from my profiles anyway.  Also he might have a copy of the 'stock' profile and not one that is part of the vendor space.  Especially if his stock profile was originally from a much older version of slicer from before they implemented the Vendor space change.  I seem to recall my original mk3 profile (non s) didnt get any of the updates after the vendor change.  I wasn't using it anyway so I didn't care.

If he checks his configurations in a text editor he could look to see if the prusa vendor space is part of them.  If he does a clean install then the new profiles are likely to be added, ie run the wizard to add the printers he wants cleanly.

Posted : 11/01/2021 3:26 pm
Honorable Member

Thanks guys, that's what I suspected. Curious why they added that part to the gcode. I did a (admittely super quick) search on github but didn't find anything.

Posted : 11/01/2021 5:05 pm
Prominent Member

I'm baffled why anybody would distribute GCode and why anybody would download it?

Posted : 12/01/2021 12:07 pm

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