Response to the home switches became erratic and the DRO would Zero occasionally when the switches were activated and not other times.
In order to eliminate the break out board I ground the pins on the parallel cable where it plugs into the BOB and simulated switch actuation at that point hoping to eliminate a number of variables.
When the symptom persisted I concluded the BOB was not at fault and the cause lay elsewhere.
Checked the BOB XML file size in the Mach3 directory it was not the same as supplied by the manufacturer so it was reloaded/reconfigured and all appears to be working as it should (and as described by TPS in his post).
System now homes as required when pressing 'Ref All Home', just need to sort out some additional settings and we should be good to go.
I'm retrofitting and old Gravograph engraving machine and by design it homes X at 0, Y at the top of its movement range and Z also at the top (as is the case with other mill set ups).
Will sort out settings for Y axis, offsets G55 etc. next.
Steep learning curve but interesting none the less.
Many thanks to all for your suggestions, no doubt will have more questions in the future.
Regards,
John