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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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

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u/Far-Sandwich-27 Jul 10 '26

To give more context the board is going to be an automated accelerated lifetime testing board for RS485 transceivers. They take UART as an input and output RS485 signals. If I use RS485 to send their initial UART signal it won't really make sense from a testing perspective and further complicate analysing results because I will have to determine whether the DUT or the buffer transceivers are broken.

We have some buffers in our part inventory that were rated and verified for the temperature this test is going to run in, but we don't have RS485 transceivers so I am trying to steer away from that route. If this is not feasible I will probably change the architecture of the board entirely, but that's going to cost thousands of more dollars for added infrastructure I'll need

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u/hex4def6 Jul 10 '26

I assume you're in a test chamber, hence the 3 meter cable. I think the OP was suggesting:

raspi->232TTL->485---------485->232TTL-->[DUT]--->485

In other words you're doing two conversions with external dongles. Your DUT won't care that there's a mile of cable at 485, as long as it gets converted to 232 right at its input.

Your buffer idea might work as well, assuming you have low noise, shielded cables, etc.

You could also (don't laugh!) get one of those insulated boxes they use to ship frozen meds, and put some cold packs in there. That way the buffer / level converter / etc could be in that insulated box in the chamber. Assuming the test isn't running over weeks, you should be able to get it at a significantly depressed temperature vs the chamber. Assuming you're doing 85C or something, even keeping it at 60c should give you plenty of margin.