Optocoulped Parallel port

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.

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
30 Griswold Street, Cambridge, MA 02138
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