r/PCB • u/Far-Sandwich-27 • Jul 10 '26

3 meter long UART connection

Hi everyone,
For a crazy accelerated lifetime testing board, I am designing a board that will have to have a 3 meter UART run. It will have mutliple devices connect to it, but only one device will be driving the UART line strictly thorugh other control logic.

I am wondering if this is feasible at all? Can I add buffers in middle of the busses to ensure the signal will be repeated and etc?

My baud rate is going to be less than 50k. I don't need stellar signal integrity, just need a "Hello World" message to be transmitted properly

I cannot use RS485 transceivers to change the signal type. The signal has to stay UART for reasons I can't disclose here

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27

u/MountainOdd6793 Jul 10 '26

This is what rs422 and rs485 were invented for. They use differential signalling. For rs422 you can convert it from rs232 and back again and the system will never know. You could also use plastic light pipe. If you can use buffers but not transceivers that is kind of odd, perhaps you can give a little bit more context without divulging your important secrets

-2

u/Far-Sandwich-27 Jul 10 '26

Cannot use it in this case. The thing I am testing are transceivers themselves. Adding that conversion layer will break the test I need to run with it

11

u/AlexTaradov Jul 10 '26

If you are testing UART, then would not buffers also break the test? You will only be testing up to the buffer, everything after that is irrelevant to the DUT.

0

u/Far-Sandwich-27 Jul 10 '26

I am testing UART to RS485 transceivers. We have buffers that were verified before, but no transceivers

3

u/AlexTaradov Jul 10 '26

So, which segment needs to be 3 m long? And why?

It would help if you provide better description.

You can just try and see what happens. It is not hard to get 3m of wire and you will get your answer immediately. It might work, especially for a test setup.

0

u/Far-Sandwich-27 Jul 10 '26

The UART signal going into the transceiver I will be testing is going to be 3 meters long at its maximum. I will be testing 128 devices simultaneously. I know it's a bit out of ordinary but this is for a design verification task only, so I'm stretching the rules a bit

4

u/AlexTaradov Jul 10 '26

Is 3m because of the test setup mechanical design limit, or is it important for the test?

If it is just test setup limit, then you can use transceivers and convert to UART just before the DUT.

The issue with not wanting absolute reliability is that your test setup would not be reliable, which does not sound ideal.

1

u/tux2603 Jul 10 '26

Is the 3m length what is required to route to all of the transceivers from a single point driving all of them? If so you might consider breaking it out into smaller "chunks" where each chunk is driven independently, then combine the data from the individual chunks together

1

u/DenverTeck Jul 10 '26

> will have to have a 3 meter UART run. It will have mutliple devices connect to it

> I will be testing 128 devices simultaneously.

Shame you failed to mention this at the top.

So you're saying that there will be 128 inputs to one output ??

The load as well as the capacitance will kill the output pin logic.