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I'd have to dig up the specs, but Cat5e is usually around mid 50's pf per meter (@ some frequency).
That's only really valid for signals with rise times much longer than the round trip time on the cable. With 50 ft and 30V/micro-second, with about 30 volts peak to peak, an RS 232 conforming signal does approximate to that constraint.
To the extent those constraints don't apply and both ends are terminated well above the characteristic impedance, the output voltage actually goes up in a staircase, with the steps being the round trip time.
Some other mistermination conditions cause a ringing approach to the final value, which is why it is better to operate with a high impedance load, and therefore capacitive characteristics.
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