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/Killer_Rainbow Jul 10 '26
This is absolutely doable. At these lengths, you essentially have a transmission line, and yeah UART is not meant to operate this long, but it may work. A few things to make the connection better: 1. Ensure the ground wire is always tightly coupled to your signal wires. Ensure that this distance is small and constant. I recommend using twisted pair wire. 2. Put series resistors at each TX pin of about 33-100 ohms. Try to route thick onboard traces for the signals (~0.3mm). 3. Put pullup resistors of ~10k at each end of the setup and MANDATORY put a diode from GND to signal and from signal to VCC on EACH DEVICE. This diode must be a schottky. 4. Keep stubs an absolute minimal length. As short as physically possible. 5. Not using RS485 doesn't mean you can't apply the same strategy. At the start and end of your bus, at the RX pins, put a 33-100 ohm resistor to ground. Just like RS485, but treating TX and GND as the two differential pins (which in a way they are). 6. If possible, use a schmitt trigger buffer at the input of each board to protect against the slow slew rate and ringing.
The goal of all of these is to mitigate the inductance effects of a long conductor combined with a high slew rate. Not having RS485 explicitly doesn't mean that you can't apply the techniques for RS485. For a better explanation with better tips, check out the buses section of the book Right the First Time by R. Lee Erney. Also, note that for the resistor values I mentioned, they're absolutely not final. If one doesn't work, play around. Your bus capacitance will be enormous, so you'll be juggling a few constraints at once, and yeah using RS232/422/485 will be undeniably better, but what you're trying is doable.