Just tried to simulate the communication line with the series resistance in it:
As you can see even at 1 MBPS the signal at the "receiver" is fairly good. I'm pretty sure that this will be OK to receive the signal.
The reason I tried to think about the series resistance is an intention to put the resistors at the reseiver with a parallel TVS (or Zener) diodes to protect the receiver (I will need to have a long transmition line). After I put the zeners the falls and rises become much slower (I beleive that it was due to diodes parasitic capacitance):
I think it would be a bad idea to put TVS without a resistors.
15 pF capacity I took from some RS-485 transceivers datasheet. Actually I was able to find the receiver input capacitance only in the TI's AM26C31ID datasheet (it was 6 pF). Probably there are other devices with this parameter specified but many from MAXIM and Analog Devices was not :(
So it looks that the wire series resistance is not an issue for communication under real circumstances. Are there any other points I am not aware of?
I made a simulation with lossy transmition line (LTRA) and came to the conclusion that the series resistance is not influence much to the signal (it is 1 MBps, at 200 meters, which is pretty long for this speed):
However this resistance CAN influence the signal in some cases:
- if the termination resistor placed AFTER there resistance
- if the series resistance become close to the input impedance of the receiver which leads to significant amplitude reduction.