Error finishing read when plasma activated

I am the final stages of commissioning my corexy plasma cutter controlled with a 7i96s.  Today I finally got the touch off code working, so moved to firing up the plasma cutter.  The M3 S1 gcode is switching the plasma cutter on (connected to outputs 4ab) - air blasts and the plasma momentarily appears before getting the error - hm/hm2_7i96s.0 :error finishing read! Iter=18747.  I have also issued the M3 S1 with the plasma switched off, no error. Any ideas? 

Thanks,
Mark