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Author Topic: Parallel port wiring  (Read 4748 times)

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Parallel port wiring
« on: November 01, 2016, 10:09:58 PM »
I bought a cnc 3020 for a specific task, not machining. It has a DB25 connector for connection to a parallel port and is set up for Mach 3. I am not using mach 3 for this task, I am using a dedicated controller. I don't want to blow out the driver box so I am wondering what input voltage and current it expects on the axis inputs?  I would guess 5 volts but I have only a 24 volt supply at the moment. If I had a 5 volt supply would I also need to use current limiting resistors in the circuit?
I am a power and industrial control guy not electronics so much.

Thanks,

Offline stirling

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Re: Parallel port wiring
« Reply #1 on: November 02, 2016, 05:20:09 AM »
I am using a dedicated controller.

got a name or better a link? - all shall be revealed.
Re: Parallel port wiring
« Reply #2 on: November 02, 2016, 10:25:01 AM »
Its called a TopCNC TC55H. The outputs have + and - terminals so I figure they are open collector and you supply whatever voltage your drives require. The CNC3020 driver box is blue in color (no part numbers) appears to have driver board for the four axis with the toshiba stepper driver chips. Don't have any wiring diagrams so this is way more complicated than necessary.  

What I am building is a waste water sampler. Use fourth axis to drive a peristaltic pump to pull a sample, move to test tube location, dose a sample into the test tube, reverse to pump out to clear out the sample tube, pause for a period of time, repeat.  All G0 moves in X,Y and maybe Z.

Offline stirling

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Re: Parallel port wiring
« Reply #3 on: November 02, 2016, 11:17:47 AM »
OK - then back to your original questions:

Yes you'll need a 5V supply because (presumably) that's what your drivers logic will need (given they were intended to connect to a PC parallel port). Does the driver box not already have a 5V supply built in that you could tap off?

If the controller is open collector then you'll need pullup resistors because when the transistor is off the open collector output would be floating.

Not sure what you were intending to do with current limiting resistors - can you expand a little?
Re: Parallel port wiring
« Reply #4 on: November 02, 2016, 02:25:02 PM »
I am trying not to hack the driver box, and only connect via the DB25 intended for a parallel port. That way if the box fails I can just replace it and move on.

The current limit resistors was just a thought about not putting 5 volt from a supply that might supply amps into a port that can only handle ma.

Offline stirling

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Re: Parallel port wiring
« Reply #5 on: November 03, 2016, 05:40:14 AM »
The current limit resistors was just a thought about not putting 5 volt from a supply that might supply amps into a port that can only handle ma.

You're making the classic mistake of thinking your PS will somehow "push" more current into things than they can "take" - When you apply a voltage across something, the current will be determined by the resistance of that "something" according to Ohms Law. (which here will be milli-amps). Doesn't matter how many amps the supply is capable of supplying as long as it's "enough". i.e. if the connected circuit "draws" 20mA and you have a 100A supply it matters not (apart from wasted money of course).

That said - without knowing about the input config of the drivers. it's not really possible to say the best way to do this. For example, IF your driver inputs are opto-isolated and are driven by SINKING current then you wouldn't need to do anything except attach your open-collector. However if they're opto-isolated and driven by current SOURCING (unlikely), it's a different ball game. If they're NOT opto-isolated then you'll need a 5V supply and a pullup.
« Last Edit: November 03, 2016, 06:02:52 AM by stirling »
Re: Parallel port wiring
« Reply #6 on: November 03, 2016, 08:21:53 AM »
Actually my thinking is more along the line of driving an LED. They usually need a series resistor selected for the voltage you are using to limit the current otherwise they will burn up.  This would be simple if I had a wiring diagram!

Offline stirling

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Re: Parallel port wiring
« Reply #7 on: November 03, 2016, 08:28:48 AM »
You're quite correct that you (always) need current limiting to drive an LED but this is the first time you've mentioned an LED (I've already asked about optos). This is why we need to know what we're dealing with. As I said above, how you go about this completely depends on the type of input we're trying to interface to. The crux is that unlike a parallel port which is (almost certainly) totem-pole, open collectors can't source current they can only sink.
« Last Edit: November 03, 2016, 12:19:40 PM by stirling »