In article , Clint Sharp
writes

Obviously, the speed is fairly important and lower speeds will work
better over longer cables but I've used 115k at ten metres and 19.2 over
some silly distances (around 35 metres for a terminal) without
noticeable problems.

I've seen several installations using 9600 baud at a couple hundred
metres, though at those lengths quality shielded cable is needed and the
line drivers are more prone to failure due to induced noise. This was
RS232, not one of the balanced variants.

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