Serial LCD Display

I found an LCD display which works off of some kind of DB-9S connector. I hooked it up to a PC RS-232 and just used a basic program to output keystrokes maybe? I set the baud rate @ 19200 and selected com6 which it was attached to. Whatever I typed on the keyboard, appeared on the LCD display. I hooked it up to a digital scope and I could see that there was a low signal until you pressed a key. You would then see a start bit followed by 8 additional bits. Looking at the 9 bits, the first and last bit were always high. I can't find what data is actually being sent by the keyboard to see if this matches but this is what the LCD wants. I hooked up the arduino and tried some things but it seems like the UART the arduino uses has the signal held high the whole time until I send data. I need it to be low with a high as a start bit but arduino sends high with a low for a start bit. I hope I communicated that well. Is there a way to make my arduino use a low signal, a high start bit, then the 8 bit number, then low again?


Yes your arduino is using TTL serial data, where the normal state is high and the data starts with a logic zero start bit. These signals are at zero volts or five volts.

Your LCD and PC are using RS232, the data is of the same format except that:- 1) It is upside down, with the normal state being low. 2) The voltage levels are -12V for normal and +12 for the driven state.

So to use your arduino with your LCD you have to convert TTL to RS232. There are many chips that will do this. One I use a lot is a MAX202.

I found this tutorial:

It seems like it is more than I need. The LCD display simply receives code. That's it. I honestly don't know why it uses that connector when all it is using is 1 pin. It is externally powered. I just think that tutorial accomplishes more than I need to. Is there a simpler tutorial that only does what I need which is to convert the TX (pin 1) from TTL to RS-232?


You are going to have to convert the voltage from TTL to RS232, so you can do it with a chip that will probably do other stuff as well or you can do it with a few transistors. However the chip has the big advantage of generating the negative voltage rail insted of you have to supply it as a separate power supply.

It may be possible to do this without a negative voltage at all. Lots of "RS232"/serial stuff is OK with +5V/0, [u]but[/u] you'll have to invert your serial output. Use an NPN, common emitter, a "switch" with the collector pulled-up and going to the display's input. It's worth a try.

The only thing I'm worried about is the timing. Inverting the signal sounds awesome although I don't know why you couldn't just use logic inverter chip like a 74HC04N or something even smaller IDK. Just think it'd be easier. But again, the timing is what I'm worried about. This thing is running at 19.2k!

If you have an IC then use that, no difference. You're "worried" about timing? Stop, it's a Non-issue. Instead of taking all this time, why not just wire it up and give it a go?