The limiting factor is the capacitance of the wiring. Back when RS-232 was king, 50ft was the limit for 9600bps. By using wire with thicker insulation and lower twist of the pair, you could get 100 ft because the capacitance was lower.
All this difference is because the signal was polar, reversing from + to - and the line drivers had to reverse the charge on the wire pair. This took time, making the signal not a square wave, but one with sloping sides.
Same applies to any serial communication over wires. Speeds were increased when driver devices could switch faster and supply more current.
So measure the capacitance of the wire pair and that will directly relate to the bit rate it will support with a particular driver.
Paul
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