USB to Parallel port converter

I see. But what if much of that work were offloaded to an inexpensive motion controller that handles all the critical timing? The heavy lifting would be done by dedicated hardware. This would remove the real-time linux requirement and the rapidly disappearing parallel port requirement. You could probably even run it on a raspberry pi for under $100. (If not raspberry, then on many other linux boxes.)

USB breakout boards are sold by Dynomotion and CNC4PC .

Unless you move HAL to the external device, you still need real time on the host
(or you lose the flexibility and extensibility of the HAL machine interface)

To be able to run HAL you need fast hardware, fast floating point, dynamic module loading, etc
so all you have ended up doing is moving LinuxCNC to the external device
(and LinuxCNC/Machinekit will already run on small ARM devices)

Adding another buffering layer (as USB requires) just makes the machine control model more complex

For less than the $250 bucks a Kflop costs (plus what ever breakout boards it might need) you can do a whole lot more with a couple of things from Mesa.

Do any of them work with an existing Linux box that has ethernet and USB2/3?

Yes there are some that use an ethernet connection. There is the7i92, 7i80 and 7i76E.
store.mesanet.com/index.php?route=produc...oduct&product_id=302
store.mesanet.com/index.php?route=produc...rt=p.price&order=ASC
store.mesanet.com/index.php?route=produc...rt=p.price&order=ASC

But it has to be a good solid ethernet interface capable of real-time (not a USB/ethernet dongle.)

Or:
mesaus.com/index.php?route=product/produ...ath=66&product_id=66
mesaus.com/index.php?route=product/produ...ath=66&product_id=99
mesaus.com/index.php?route=product/produ...ath=66&product_id=98
mesaus.com/index.php?route=product/produ...ath=63&product_id=76

JT