input to PC parallel port

Not knowing the details. Here is my guess: You are comparing against a 10V signal (21K,10K divider). So, your LM339 is at least 10V VCC, and the output is almost 10V.

Check if you parallel port is still working (not smoking).

I tried it with a 5k-25k voltage divider as well - no good. And it's not smoking... How do I check whether the comparator is "strong" enough to raise the pin to '1'?

Try it without the comparator, by connecting the pin to +5V and ground and checking that you can read the values with your software. Then check that the comparator is working by testing the output with a DVM or scope.

Leon

Without the comparator (Directly from a power source ) it reads ok, although only from about 7V. I was thinking maybe the 20k and 10k resistors are too large. could this be a reason? Thanks

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l>

\\I tried it with a 5k-25k voltage divider as well - no good. And it's \\not smoking... \\How do I check whether the comparator is "strong" enough to raise the \\pin to '1'?

If you tried to push 10V onto a parallelport pin it might as well been blown. What pin did you use anyway? What mode is the parallel port suppose to work in? For a comprehensive treatment of PC ports see:

Fisrt, you should select in your bios setup SPP for the printer port. thats short for Standard Parallel Port. That ensures that the outputs and inputs behave like standard TTL . Then if you want some software and info to horse around with, get laplink.zip at

The LM339 is opencollector and requires a pull up resistor to +5 volts.

Dave

A simple setup below.

Looks like you have a problem with that input. It should respond to TTL levels.

Leon