What is the function of a parallel port breakout board?

[Jonathan]

I'm looking at some available breakout boards now, and some feature the charge pump as some form of safety. How to the breakout boards implement this?

It's generally a circuit that enables the outputs, via some logic gates, when it detects a square wave of the right frequency on one of the port outputs. That square wave is outputted by the software, so the outputs are only active when, in your case, LinuxCNC is running. It stops the motors moving a tiny bit when you boot the computer, that's about it. I wouldn't worry if the breakout board you have doesn't have it.

Also, regarding the use of more limit and home switches than available inputs, what's the most common solution? I saw the documentation on LinuxCNC which suggests connecting the switches in series. This doesn't seem to make much sense especially if the machine starts in a completely unknown state.

So long as you only press one (normally closed) switch at a time, then the software can use it for homing since it know which axis is moving, and thus which switch must have been pressed. If that still doesn't get you enough ports you can add a second parallel port - generally a PCI parallel port card.

[hbx]

Ah I see, but this only works if only one axis is moving? During operation when everything's moving and something hits the limit switch, how will the software resolve this?

[Jonathan]

Ah I see, but this only works if only one axis is moving? During operation when everything's moving and something hits the limit switch, how will the software resolve this?

It will just stop the machine, since clearly there's a fault, and it's up to you to resolve it - i.e. move the relevant axis off the limit switch.

[hbx]

I suppose that's reasonable! Thanks! I may look into using something other than the parallel port. For LinuxCNC users, what are the next best options? PCI cards?

[Clive S]

Just a little note regarding the PP BOB it can also act as a level shifter as most PPs these day are 3.3V and the drivers seem to work beter at 5V so the BOB can convert the 3.3V to 5V. Clive.

[Swarfing]

For Linux have search on Massa cards and network ports as an alternative?

[hbx]

I've been taking a look at the Mesa cards, and the 5I25 looks nice and simple. I'll stick to the LPT for now, while I figure out how to set LinuxCNC up.