Printer Port Opto Isolation

[jophuds]

Hi,

I'm trying to use my printer port as a Digital Output Interface to a motor control IC. I want to use an opto-isolator circuit to protect my PC. However, the printer port outputs a 3.39 Volt High signal and a 0.1 Volt Low Signal - this is not enough to switch the opto-isolater by itself and so I need a 5 Volt reference voltage from my PC.

Does anyone know a simple way to get this?

Regards,

Joe

 

[itsmoked]

It just sucks they didn't include 5V on the printer port.

You have to bag 5V from inside the comp and run it out. Get a HDD power cable and hack it to provide the GND and 5V. Twist these leads and run them out to your controller.

Note: Highly risky. You touch that 5V to anything and you can lunch your computer totally. You can fry every device in it including the CPU and the Motherboard. I would recommend an 1/8A fast blow in-line fuse.

This whole scheme is rickety because you end up with nasty ground loops because you bring out the TTL printer signal far removed from the 5V & ground.

I would not recommend this setup permanently but as a trial sure. It may work just fine. I use this to run my CNC router on a permanent basis. But it's an old PC-XT junker.

As for how an opto is going to work... It is pointless if you don't have a separate source on the far side of the opto.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[benta]

Why is over 3 V not enough to swich on an optoisolator?
The LEDs in the couplers I know need just 1.4...1.5 V.
I think your problem is current transfer ratio plus too heavy loading of the output transistor.
The printer port will only supply a couple of mA for the LED (remember a current limiting resistor!). If you have a coupler with CTR of 100%, your output transistor can only have a pullup that's corresponding to this current.

Next problem is, that when you run optocouplers at this low current, they get reeaal slow.

Benta.

 

[geekEE]

You might be able to steal 5V from the PS/2 keyboard/mouse connectors (assuming that your PC has them). It's not high current, but it'd probably work for you.

I seem to recall that most printer ports can sink a fair amount of current, but are bad at sourcing current. I am surprised though that you can't use them to turn on an opto. I used to use them to drive LEDs all the time for debugging purposes.

 

[felixc]

Parallel port interfaces are more working as current sinks than current sources. (the original spec was using TTL circuits) The printers usually had rather strong pullups to compensate for the cable capacitance. As someone else suggested, you can use the 5V from one of the mini-DINs. In the past several peripherals that needed a power source were tapping it from a PS/2 port. Using this 5V source, add a series diode to reduce the voltage down to some 4.4 volts, and use your optos in sink mode. In this mode your parallel port interface shall have enough current to properly drive the optos.