Hi,
This is the first time I have ever posted to a forum, so I'll do my best not to write too much.

So, I've got this old LED display, and it receives text to display on it via RS232, and ofc I just couldn't get it to display anything... but in my efforts to beat this dumb display i have actually been able to acquire and read the data sent to it by the device it originally came from using a laptop with a usb to rs232 adapter cable, it receives ASCII messages with stx/etx either side at 9600 baud, odd parity. I can read all the characters clearly.

What's weird is, even if I send the display this exact information via realterm, on the laptop, it doesn't display anything.

The pins are correct (it's only the basic tx rx-- 2,3, and 5(9-pin), but I'd did swap 2&3 anyway just because, I also checked that the display still works on the original device. If I loopback, real term receives the signal it sent so I know my cables are good.

If anyone knows why this doesn't work I'd be very interested because I thought this was weird. Likewise if I figure it out I'll post what I've done wrong

 

Many RS-232 devices rely on control/handshake signals, in addition to the TX and RX pins (2, 3).

Take a look at the RS-232 in Minimal Handshake Applications section here:
https://www.analog.com/en/resources...damentals-of-rs232-serial-communications.html

Try connecting the RTS and CTS signals (pins 7 and 8) between the display and your USB to RS-232 adapter.

 

Hi, thanks so much for replying. And the link is a great resource, much appreciated!

Unfortunately confusion still continues for me as the display really has only the 3 minimum pins And entirely lacks the use of RTS and CTS. on the inside of the display, the PCB is only connected to those three wires from the connector and power 0v and 24v supplied from another separate power only connector. It's seems to use the max202 ic as the drivers.

 

Hello,

To have a correct result, the baudrate, number of bits , parity bit and number of stopbits must be correct.
Read this page for more info:
https://www.arcelect.com/rs232.htm

Bertus

 

I'd caution against assuming that because you don't see a trace on a PCB that there is no connection. PCB's can have multiple internal layers that may have trace routing that you would be unable to see through the outside layer.

Are you sure the ASCII messages being sent by the device that came with the display use Odd Parity and not No Parity with Two Stop Bits?

Just to confirm I understand, you have this display and some other device that originally came with it. This device can successfully send messages to the display and the text is shown on the display. You are trying to duplicate this behavior using a PC to send messages to the display, but regardless of what you try, the display does not show any text sent by your PC. Is this correct?

If so, how have you connected the display to the device that came with it? Are you using a standard DB-9 serial cable (either a straight-through or null modem cable)? Or are you manually wiring pins 2, 3, and 5 between the two devices?

Are you using the same cable between the display and your PC's USB to RS-232 adapter?

 

Thank you for the warning about layered boards,
the PCB from the display terminates in a 5way connector that leads off to the 2 connectors I mentioned and the display does work on the original device with no other connections than those. 2 wires for power, 3 headed to the rs232 9pin sub D connector.

I am definitely sure about the settings as I can read that data as it was originally sent in realterm on the laptop but it only produces information that makes proper sense with the settings on odd parity. Etc.

Your understanding of the situation is completely correct, basically I took this display off and I really want to use the display somewhere else for a project or something. Also I'd like to be able to test other displays like it to see if they are working without the original machine.

Apologies for lack of pictures to back this explanation up at the moment, but yes I am using the same cable (it's attached to the display), and I have some subD breakout connectors I was able to use to wire the configuration of 2&3 manually to make sure it didn't work on either null or straight ( it read the data in null so using the same breakout I expected it to send in straight but checked both to be safe and neither works) and I can receive my own signals that are sent from realterm perfectly fine through the breakouts or even to another pc through another usb converter so Im definitely sending identical data too.

For added flavour, I got these breakouts a while back to connect the usb adapter to an unrelated 25pin sub D peripheral (that also weirdly enough only used 3 of the pins ).

My plan for tomorrow was to try replacing the display's cable just in case it has an intermittent break in it or something - I'm not hopeful xD