Mesa 7i76e, step dir, homing with index signal.

Hallo,
I have a linuxcnc 2.9 installation with Mesa 7i76e controlling 
​​​​4 leadshine el7 rs servos via step dir. The encoder input of the 7i76e is used for the spindle.
Homing without index is working perfect.
Is it possible with current hardware to home with index.
I can read the short index signal with Hal Scope. For a & b i have no inputs left.
I tried with software encoder in hal but homing ignores the index and axis moves on.
Do i need a firmware update for the 7i76e like a read a lot with stepgen and index homing?
Thank you

 

This can be done with custom firmware
and some kind of daughter card on one of
the 7I76Es parallel expansion connectors

If the signals are 5V a daughter card may
not be needed but some protection from voltage
spikes like  220 ohm series resistors on the inputs
is advisable.

Another possibility is for me to add a software index pin for the stepgen/encoder
in LinuxCNC's driver so you could connect say a 7I76E field I/O pin to the software
index pin in hal.

This function could be built from hal components but it's a bit complex.