Printer Port Opto Isolation
Printer Port Opto Isolation
(OP)
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
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





RE: Printer Port Opto Isolation
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
RE: Printer Port Opto Isolation
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.
RE: Printer Port Opto Isolation
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.
RE: Printer Port Opto Isolation