I made a schoolboy error - I had a fault and fiddled in two areas - powered it up again, realised I had fixed the problem but now don’t, of course, know why!
I’m sharing the problem just in case others have had it, and maybe then we can jointly chase this problem away. Here goes…
770MX running super well with Microarc4 and I have been doing some work in 3+1 successfully. Suddenly out of the blue, the fourth axis begins to “under-rotate” causing crashes. And it’s slowed down too. A G0 A180 instruction actually gave a turn of approximately 25 degrees (measured by me with a protractor). And it turns much much slower.
After powering fully down, I did two things. I know I know 
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On the back of the stepper driver control box there is a 8 position DIP - SWITCH. I flicked each one back and forth to “wipe” their contacts.
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Then I noticed the microARC’s cable, where it plugs into the electrical cabinet, was warm. I unplugged it, squirted some electrical switch cleaner into both sides of the plug and socket and re-made the connection a few times.
Powered up the 770MX again and all was well. I don’t think the plug/socket should get warm, but maybe it does on some machines. Does it? I am finding it difficult to believe this is the problem, because why would it make the microARC run slowly and not turn enough?
If it was a bad DIP SWITCH connection, I can imagine the microARC being placed in the “wrong mode” and this counting the wrong number of pulses.
So what do others think it was that fixed it? And is this a fault anyone else has seen?
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