Re: Optocoulped Parallel port

stratton@... wrote:

> From: stratton@...
>
> On the subject of driving optocoulper LED's off the parallel port:
>
> 1) I've put visible LED's directly in the port (no resistor) and had
> them light on every machine I tried so far.
>
> 2) Optocouplers are available down to 1.6ma activation
>
> 3) The Intel developer website mentions that the external connectors
> on their motherboards are 'overcurrent protected'. Intel doesn't make
>
> I/O chips and refers people to SMC or National instead.
>
> 4) National lists varying drive capability from 2 - 15 ma depending on
>
> the pin. But it also says most of these pins are open
> collector(drain) and should have 4.7k pullups (presumably on the
> motherboard).
>
> 5) 5v/4.7k = ~1ma or not enough to reliably activate an optocoupler
>
> My personal feeling is that directly driving an optocoupler will
> probably work and is okay if tested on a case-by-case basis, but it's
> probably not a good way to design a product.

You don't use the pullup resistor to power the optocoupler. You use
either the PC
power supply or an external +5V power supply, through a resistor,
through the
LED or optocoupler, and finally to the port output, which will light the
LED when
the output is at logic ZERO, or grounded. More current is available
that way.
This all applies to output from the PC to external devices.

Jon