Ignoramus6355 wrote:
The more disconcerting news is that if a tach cable is not connected,
the drive goes into a runaway mode EVEN IF NO INPUT SIGNAL is
present (inputs shorted to each other).

Yes, of course. Any servo needs to have its "loop" closed at all times.
It may be that if you put a 10 K Ohm resistor across the tach input
terminals on
the servo amp, then an open tach circuit will no longer cause a runaway.
This means that if I do a shoddy job wiring those tach contacts, or
the open barrel terminals fall out, or anything else, I will be in a
world of hurt.

Well, the tach can also have the brushes go bad, the wire can fail due
to flexing, etc.
I know that EMC has some protection against following error, but I am
not sure how robust is that.

It is unlikely the tach AND the encoder will fail at the same time.

I just got in the boards for my "Copley interface", which will also work
on the AMC amps
(they are the same). The servo amp needs a contact closure to ground to
enable it, and
produces a 5 V signal when it goes into fault status. A quirk is that
when disabled, it shows
fault status. So, this board suppresses the fault indication for a
moment until the amp has
gotten enabled. The signal from one of the DIO SSR's can be used to
send the enable to the
servo amps. By default, EMC2 does not go to E-stop on a following
error, allowing the
servo amps to drift (or continue to run away, if that was the cause). I
have to find out
how to modify the ppmc_io.hal file to make this happen.

Jon