Hi, I've been trying to read a signal by a parallel port. The signal is the output of a comparator LM339, by a 21k resistor connected to 30V, and then 10k connected to ground. The problem is - whenever I connect the parallel port - the signal dies (0V read). What am i doing wrong? What should I check? Can anyone refer me to an example circuit where data is read by a parallel port? Thanks!
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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\\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.
+5------2K resistor ------output of LM339 and input pin to port.Dave
A simple setup below.
Looks like you have a problem with that input. It should respond to TTL levels.
Leon
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