What is the function of a parallel port breakout board?

[martin54]

It is as you say an easier way to gain access to the port pins. Loads of different boards available, some with opto isolation some without, same with drivers although as you say a lot of drivers now have them built in.
Most boards will not only give you an easy connection to the drivers but also have other functions like spindle isolation, inputs for things like E stop & limit switches all using different pins.

[hbx]

Thanks! Do the boards somehow increase the number of inputs available? Perhaps by multiplexing? I'll be using LinuxCNC, and the documentation doesn't seem to be very comprehensive.

[Jonathan]

When you use drivers that are already opto-isolated, then it's sensible to use a breakout board which doesn't have opto-isolated outputs, since as you say there's just no point doing it twice. In fact, since the drivers are isolated it's perfectly safe to connect them directly to the port and not use a breakout board, however you would still need to make an isolated circuit for the inputs, and it sometimes doesn't work if the port voltage is too low, so it's generally not worth it except for testing.

Thanks! Do the boards somehow increase the number of inputs available? Perhaps by multiplexing?

Unfortunately not.

[hbx]

Thanks. Isolation the inputs sounds logical. 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? Is this meant to be detected by the software?

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.

[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.