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Sporadic interference on an open collector bus, at 2.5MHz?

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Artlav:
I got a single wire UART bus spanning dozens of meters, and things on it tend to glitch and lock up a few times a week.

I've tried adding clamp and TVS diodes and series resistors, to little effect.

Eventually i sat there staring at a scope for a few hours, and was able to capture several bursts at 2.5MHz, about 4 times an hour. They vary in duration and intensity, but are fairly consistent in frequency.

What is it, where can it be coming from and how to protect from it?

I suspect weather might have something to do with it, since it's much rarer in the winter than in the summer, but i don't see where the frequency comes from.

I'm also near an airport with planes flying over, but the bursts are there even at night when no one is flying and don't seem to coincide with the planes in any obvious way.

EDIT: Fixed images





minifloat:
You're saying single line uart... Did you use some kind of screen to protect it against EMI?

Multiple times an hour, I think it could be
• ripple control signal
• CCFL or FL start
• something else regularly occurring

Summer more than winter, maybe a starter device of some aircon or exhaust ventilation?

Do you have some galvanic isolation on the line TRX of the bus nodes?

br, mf

tggzzz:
An open collector connection usually means relatively high impedance, so a small amount of interference energy can produce a larger voltage. Try reducing the pull-up resistor.

You don't say what the transmitter and receiver and cable are, so we can't assess whether that is sensible. However a series resistor at the source seems odd with an open collector bus.

Dozens of metres long. The ground/neutral potentials at each end could be changing, e.g. if a motor or heater switches on. Isolate the grount/neutrals by using optical fibre or an optocoupler at the receiver.

David Hess:
If it cannot be screened and there is no ground loop, then the easiest solution may be to limit the bandwidth at the receive end to a level commensurate with the baud rate.

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