Homing dependent axis

I'm currently in the process of working on a 4-joint 3-axis drawing bot to be installed at the local science museum. As it needs to run unattended for ~12 hours a day it needs to automatically home itself a few times a day. From what I can tell homing a machine with independent axis is trivial, however since all 4-axis on mine are tied together I need positive feed on 3 of the joints and negative feed on the joint I'm homing for it to work correctly without slipping.

I have a working C program that talks directly to the parallel port but it would be far more useful if it were all done through linuxcnc. I have experimented with rt hal components to try and achieve this to no effect. I'm currently thinking that I will need to write some kind of user level program that talks directly to motmod but I have been unable to find a good example of how to attempt that.

Any suggestions, either for where to look to get started or an alternative approach I haven't considered yet?

hmm I wonder if you could use a custom M or G code to call your C program ( or convert it to a comp program and use HAL )
then just tell linuxcnc to consider where it is as home.
You can home and unhome using halui pins.
Just trying to give ideas as I have never done anything but Cartesian machines.

Why do you need to home so often? Why will it not stay in synch?
Also if the joints don't move when shut down you can make linuxcnc just remember where it was...

Chris M

Thanks, halui did the trick