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From: "Roger Lascelles"
Subject: Re: PC parallel port - isolation?
Date: Wed, 18 Dec 2002 18:17:42 +1100
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
You often do need a separate power supply just to drive the optocoupler
LEDs. It can be a simple DC output plugpack which you can have separate or
wire inside an equipment case. Small DC-DC converter modules are also used.
Alternatively, you can use the PC power supply.
In order to avoid all those optocouplers, I prefer the serial port for
device control and place a microprocessor in the controlled device. That
way I only need 2 optocouplers. If you don't want to provide power, the
Serial port can drive a sensitive optocoupler LED directly, or you can get
power ( 6 to 9mA when combined thru 2 diodes ) from the DTR(pin 4) & RTS(pin
7) - these go +ve when you assert DTR and RTS.
"Jem Berkes" wrote in message
> I've been reading about the PC parallel port and understand that it uses
> something similar to the 74LS374 at its outputs to provide TTL signals
> some buffering.
> What I'm unsure of whether I can optically isolating the parallel port
> signals (which are off the computer's power supply) from my own external
> device. The parallel port output data pins can sink 20 mA but the control
> and status lines can't do that much.
> Also, if I'm providing that external voltage supply that's powering the
> LEDs in the optocoupler and the parallel port is sinking, I'm not really
> isolating safely right? I think to truly isolate, the parallel port would
> have to drive the optocoupler LEDs itself which I don't think it can do :(
> Jem Berkes
> Windows, Linux & UNIX software
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