LCD module - PC Parallel port

should be less then about 0,5V. You must have a short circuit somewhere...or your port is defective. Check your wiring.

Thats good.

HTH Wolfgang

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I don't know if the parallel port wired correctly, but the resistor on pin 3 of the LCD should be a 10k pot, wired between +5 and GND and the center of the pot to pin 3 of the LCD. Right now you have a fixed contrast and you want it variable. rw

I checked connecting it to another machine. Output voltages are the same.

O'kay, How can I make the voltage 2.48 -> 0 (for Logic Low) and 5.5 ->

5 (for Logic High)? So, that it comes in the acceptable range of HD44780 LCD module?

Thanks,

-Neo

rw is right, and this could be a contender for the '16-black boxes', the contrast is too high. Have you used the circuit with the pot (incorrectly wired in the diagram as rw has already mentioned), or the circuit without the pot. My recommendation...you need the pot (not all LCDs behave the same in my experience).

Also, how did you measure the 0V level?. Did you measure it when the PC was attempting to write to the display? or did you measure it when the PC was idle?. If you have a scope it would be better to capture a data burst and go from there.

Don't shoot me down, but you did measure the data port signal with respect to your parallel port 0V and you have connected the parallel port 0V to your

5V supplys 0V? It doesn't show this connection on the schematic. If you are using an isolated 5V supply (a battery?), then you do need to connect the parallel port 0V to your 5V supply 0V line.

Jim

By connecting the ground ?

Meindert

hi, which pin is the ground on the PC-printer port? That schematic shows nothing. rw

got the answer: any of the Green pins (G18, G19, etc)on the printer port is GND. So pick one of those to connetec to the power supply GND to and you have the proper reference. rw to your input levels.

The problem was solved when I connected parallel port GND to +5V power supply GND.

Now, It's rocking!!!

Thanks a lot to all of you.

Just to understand - why this was a problem, I thought GND is always the same whether it is from parallel port or from the power supply. Isn't it the potential difference, and that should be the same in either case?

I really appreciate and thanks to all who shared their ideas and help me make it rocking :-)

Cheers,

-Neo

when you add a power supply to a system (your pc) they must have a common reference point (GND) rw