LinuxCNC knows almost nothing about your hardware, you can map the XYZABCUVW G-code words to the positions of any motor that you want, they don't have to be in any particular physical orientation.
What kinematics are you using? Trivkins or something else?
Up to Linuxcnc v 2.7 the joints were mapped directly to axes in a specific order. From 2.8, the joints are independently mapped in an arbitrary fashion as you determine with the coordinates=xyzw statement in the ini file
Aciera wrote
Seems you are running a rather old build of Linuxcnc 2.9 that does not allow for custom axis allocation. Problem is that more recent builds require python 3.7 now and your system is still on 2.7. Not sure if your debian stretch can be updated to python 3.7. More recent debian releases that come with python 3 require a 64bit processor and yours seems to be an older 32bit.
The real mystery to me is why you need to define an A axis for the B and C axis to work because clearly there are simulation configs that run B and C axis just fine without an A.
Might be worth to start a new thread asking how to run a config with B and C axis without defining an A axis before changing the computer and everything. Possibly I'm just missing something obvious but I have run out of ideas
Is this answers your questions?.
$ git show 9a26e69fe
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Date: Tue May 11 13:15:17 2021 +0100
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$ git log src/emc/kinematics/5axiskins.c
commit 135b0ce5220bef8f8a34b6825723108b8b9740f1
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Date: Sat Jun 19 05:00:00 2021 -0700
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support for 5axiskins (bridgemill), refactor to:
1) support module 'coordinates' parameter and allow duplicate
letters for multiple joints for an axis letter
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The deb at commit 9a26e69fe (11May2021) precedes the commit
19Jun2021 (135b0ce52) that adds support for the coordinates parameter
