Where to start?

gandalf69 wrote:

The servos have a 2 wire power input, an analog tacho output for speed feedback, and a sine wave shaft position encoder for position calculation. I expect to need to change the shaft encoders to digital.

Not necessarily. Resolvers are actually very good feedback devices if you can get them to work. They are tough, reliable, accurate and absolute.
I am working on a way to read them with an Arduino (in fact, I ordered 10 PCBs yesterday). A better/more professional approach would be the Mesa 7i49, though I am not entirely sure how well that interfaces with EMC2 at the moment. I suspect that, with the correct firmware, it just appears as a set of absolute encoders in HAL and EMC2 is none the wiser.

Note that Pico also do a Resolver converter pico-systems.com/resolver.html

The info (5i20man.pdf et al) seem a bit light on, no block diagrams, etc, is anymore info supplied with the board?
Or maybe I should ask whether the info supplied was sufficient for you to make all the connections without blowing it up:)

The problem is that the 5i20 by itself doesn't really do very much. It only becomes useful with firmware downloaded to it, at which point it can be all sorts of different things.
The best documentation on what the 5i20 et al do with the EMC2 drivers is probably here:
www.linuxcnc.org/docview/html/drivers_hostmot2.html
Though some other capabilities are due to be released in the next revision (which is fairly imminent) and are included here:
www.linuxcnc.org/docview/devel/html/man/man9/hostmot2.9.html

gandalf69 wrote:

How many slots? If it is hundreds then you can probably just use the same encoders, if it is dozens then it is rather more difficult. Converting from analogue voltage to high-resolution position is possible (An Arduino has 6 analogue inputs, so could do the job for $30 and some software writing) but is something of a project all of its own. Swapping to high-resolution encoders is probably easier, unless the drives are dependent on the sinusoidal version for something.

It looks like about 200 but there are probably multiple outputs, they have a multiple angled mirror arrangement. I assumed I'd need to change them simply because they are analog and old and digital ones are cheap. The Arduino looks pretty neat.

It isn't so much a linux thing, as a systems-engineering thing, I suspect. There is a configuration wizard (pncconf) which takes most of the pain out of configuring Mesa hardware.

That's a relief:)

Yes, you can boot your existing PC from the LiveCD and play around with the simulator configs that are provided. All without making any changes to the PC at all.

Boot from the CD (you might need to change a BIOS setting to enable this)
Choose CNC-EMC2 from the applications menu.
Pick a sim.... Config.
reset e-stop (F1), machine power on (F2) and play around.

I have EMC2 on my PC, thanks for the instructions, I'm a bit slow to begin with. Great, now I can see what code is doing what:)

gandalf69 wrote:

It looks like about 200 but there are probably multiple outputs, they have a multiple angled mirror arrangement.

That's rather a low resolution to use as digital encoders, so unless you are looking for a fun project, swapping them to 1000+ count digital encoders looks like the best plan.