I just observed very interesting behavior, while trying to go to a particular location.
Did not expect this. Is there rhyme or reason to this?
Thanks, Eric T.
What does the log show and why do you need /G90?
Re G90: I thought I had to specify whether to do quick move, versus feedspeed move.
Log Contents:
N0 M5
ok
N0 G4P0.005
ok
N0 G4P0.005
ok
$h
ok
N0 G4P0.005
ok
G90G0X-106.125Y-298.850
ok
$#
[G54:0.000,0.000,0.000]
[G55:0.000,0.000,0.000]
[G56:0.000,0.000,0.000]
[G57:0.000,0.000,0.000]
[G58:0.000,0.000,0.000]
[G59:0.000,0.000,0.000]
[G28:0.000,0.000,0.000]
[G30:0.000,0.000,0.000]
[G92:0.000,0.000,0.000]
[TLO:0.000]
[PRB:0.000,0.000,0.000:0]
ok
$G
[GC:G0 G54 G17 G21 G90 G94 M5 M9 M56 T0 F0 S0]
ok
M56P0
ok
$J=G53X-6.4750Y-10.1000Z-3.0000F1500.0
ok
That line puzzled me so I tried, and… I reproduced the issue on my machine.
I suspect it has to do with the fact that the leading “/” in the MDI command indicates that CM should pass the commands straight through to GRBL with processing them, so CM does not see the G90 (absolute mode) command, and probably think it’s still in G91 (relative mode), but GRBL is now (still) in absolute mode.
Somehow this throws off where CM things it is for jogging, and when you tell it to go 1mm to the left, it takes what it apparently now thinks are the current coordinates X=-5,4750 and Y=-10.1, substracts 1mm to X, and generates that jog command towards X-6.4750
And sure enough, I retried but this time I sent “/G91” after “/G90G0X-106.125Y-298.850”, and then jogging 1mm to the left works as expected.
Moral of the story: if you cheat on CM by talking to GRBL behind its back, be sure to clean-up and leave things as you found them, before it comes back 
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