I can't help you with as I don't have the hardware. But I can put you on the right track!.
Homing is managed in Linuxcnc by homing.c
github.com/LinuxCNC/linuxcnc/blob/master.../emc/motion/homing.c
Master branch has recently released the homecomp.comp compoonent
linuxcnc.org/docs/devel/html/man/man9/homecomp.9.html
and the source file here
github.com/LinuxCNC/linuxcnc/blob/master...onents/homecomp.comp
The purpose of homecomp is to give a method to override homing.c behaviour to build a custom homing behaivour
You should also look at Domminic Braun's ethercat cia402.comp and supporting hal file
github.com/dbraun1981/hal-cia402
which predates homecomp and also attempts to deal with hardware which homes externally. He also deals with the conversion of motion units between Linuxcnc and external hardware. Look at how he uses read and write functions. This might be able to be done in halcomp as there are procedures that read all pins and writes all pins.
Your homecomp needs to be able to deal with following errors that will ocur when the extenral drives move without lcnc's knowledge. This can be done by equating the feedback position with current position during external homing.
There are a large number of homing states that can be skipped or ignored when external homing is done so you homecomp basically just needs to initiate homing and wait intil homing is completed ,Tell Linuxcnc you are homed, and then let Linuxcnc continue on.
God luck with this, its complex because in a homecomp, you need to deal with all joints at a time where a component like cia402.comp has a seperate instance of the component for each drive being homed...
some time left after that - but i continued working on that. firs a short update on current status:
-> for homing 2.9+ is necessary (took some time that i recognized that i was still on 2.8. on my raspberry)
-> on my Raspberry the user COMP does not work on 2.9.2 (first i tried the bookworm image of dec23, and also tried to run on Buster with manual install based on instruction and froum thread)
-> anyway my pokeys.comp seems to work fine on a laptop using the "linuxcnc_2.9.2-amd64.hybrid.iso"
-< for the problems due to not yet created pins when halfile tries to write on them, i created two dummy components that add some millisecond delay during loading of Hal (e.g. loadusr -W start_delay_5000), these use the extra_setup option during start to wait for 5000 ms or 3000 ms.
i created a first version of pokeys_homecomp.comp. As pokeys.comp itself is userspace and the homecomp is RT, as a first step the homecomp is a dummy, which receives the hal-commands and routes it to the pokeys.comp using pins and signals.
-> it seems to compile fine BUT when i try to load that in [EMCMOT]HOMEMOD=pokeys_homecomp i get a cyclic errors like "USRMOT: ERROR: command timeout", so i added some debugmessages in hope that it helps to find out which function is causing that.
-> so far no idea yet.
on a second path i try to get that poKeysLib usable in a RT. Currently it compiles and EMC was starting but it did not show up in HALMonitor. But since i added references & calls for PokeysLib i get a "undefined symbols" when i try to start a hal that contains that comp.
-> here i assume that pokeyslib is not yet ready to run on RT (i assume because lubusb1.0) so i started to create a copy of that H file which excludes that (using the #undef POKEYSLIB_USE_LIBUSB), but it seems that there's more work on that to do.
