Very basic questions

UKenGB wrote:

Ah, so parallel port is not ideal and there's actual purpose designed PCI(e) cards to overcome the parallel port limitations. So that means the PC needs a spare PCI slot. That's what I was trying to determine.

Well there is also the option to go with ethernet connected Mesa boards. 7i96 and 7i76e are popular for steppers but for servos, you might use a 7i92 and a 7i77 (hope I'm right on the 7i92)

If using ethernet, does that mean a point to point connection between the PC and the external (Mesa) board or can they both just be on the same LAN?

Yes, its point to point and requires the PREMPT_RT kernel but there is an ISO here now
www.linuxcnc.org/testing-stretch-rtpreempt/
You are lucky. I had to compile it from source!
There is also a thread about using Linux Mint that is pretty current.

So on my USFF PC, I had to add a USB wifi dongle which I use for internet connection. Linux is good with sorting out the routing. There will be info on this but my wired NIC is fixed at 10.10.10.1 and my Mesa 7i76e is set for 10.10.10.10 and it worked perfect right from the beginning! You just need to power up the Mesa card before running Linuxcnc or you get an error becasue a card is not found. But once its up and running, there is no real difference between ethernet and a PCI/PCIe Mesa card.

Ah, that's a thought. Use a USB dongle for regular network and the built-in ethernet port for machine control. No extra cards needed. That would certainly be a solution for the PC with no spare slots.

Is it possible to use 2 independent connections for different controls, so e.g. split duty between ethernet and parallel port?

What about a single PC to control 2 different machines, e.g. mill and lathe - not at the same time of course?

UKenGB wrote:

Is it possible to use 2 independent connections for different controls, so e.g. split duty between ethernet and parallel port?

You could but you wouldn't. Most of the Mesa cards have an expansion port that is pin compatible with parallel ports so you would add a Breakout board there. (7i76e has 2 and the 7i96 has 1) But also they have a smart serial interface which allows you to add additional (almost unlimited) daisy chained I/O ports.

Take it to the extreme, a 7i76e could also support 2 x 7i76 cards to give you 15 stepgens plus too many IO ports for me to count before we even think about the smart serial interface expansion.

UKenGB wrote:

What about a single PC to control 2 different machines, e.g. mill and lathe - not at the same time of course?

Yes of course. The Linuxcnc chooser allows an unlimited number of configurations