Thanks for all your help.
I had pretty much narrowed it down to PDMX or HiCON but then, decided to buy a CSMIO-IP/S.

The reasons were mainly faster delivery (to the UK) and cost.  Also, since the machine was already running CSMIO-IP/M, the re-wiring would be minimal.  In practice it only took 30 mins to get it up & running.

So far, it looks good.  My problem with jerky servos has vanished.  I think the IP/M couldn't deliver pulses fast enough.
The only negative seems to be that it doesn't (obviously) control the spindle as a servo - just as a simple VFD.  It clearly has some speed regultion / calibration inside, as the spindle speed is spot on but it's not regulating it on the fly so the speed changes with load.

Rigid tapping works OK with small taps - but because there's no speed regulation, it's easy to stall the spindle on larger ones.  I'll have to see if I can tweak the servo parameters to fix it.

My next machine will be HiCON though for certain!

it seems to me you can control the spindle with step / dir signal via OB output. ask cslab for confirmation. what plugin have you installed? I have on the 3.205 I am fine. 3.305 was giving me system stability problems.

You're right, you can - and that's what I use on my Mill.  However, my servo spindle controller is an old Fanuc which only has 0-10V + Direction.
I talked to CS - and they said it was impossible.  Unfortunately, I don't believe in impossible!

Si

I'm running an ESS on mine with no issues.