Question

LPT (parallel port) voltage and current

By steve19814 ·
Tags: Hardware
Hello.

I am wanting to re-purpose an old laptop computer to use its parallel port (LPT printer port) to output a +5V/0V signal on one of the data pins. The load requires about 9 mA at 5V.

I know how to program this; my question is about the LPT hardware.

I hooked up a data pin to a simple circuit with a variable resistance load, an ammeter, and a voltmeter... and got the following mA vs Volts at the data pin by varying the load resistance:

Volts mA
4.53 0
4.23 0.15
3.97 0.28
3.93 0.3
3.78 0.38
3.5 0.38

From this data, is there any way to identify the output driver chip(s) being used in the LPT port?

I've read two books:
- Parallel Port Complete by Jan Axelson, and
- Interfacing the PC for Process Control by Dhananjay Gadre

... and the data I got above is not consistent with the information in either of those books.

Thank you.
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