As far as i know rs485 specifies a bus common mode
voltage of: -7V to +12V
the terminating resistor is 120ohm , using VxV/R , yields high power rating to the resistor , certainly more than 1w . (12×12/120 =1.2w)
However looking at many products and evaluation boards , i see that they use 0805 resistors which are about 0.25w only. They might be pulse withstanding type though.
What am i missing here ? the only explanation i could come up with is that the continuous power rating of this resistor is much lower (so my calculation is wrong), since there is some break between transmitted bytes.
But in this case what happens if transmission is continuous?
Best Answer
I suspect the answer is in bold in the first line of your question. The resistor is between the data lines but the differential voltage is only a few volts - the common mode voltage can be anything but won't cause an increase in potential across the termination resistors.
Figure 1. Extract from Linear's TIA/EIA-485-A Standard.
The table above says that the limit is ±5 V. Rerunning your calculation gives \$ P = \frac {V^2}{R} = \frac {5^2}{120} = 0.208\ \mathrm W \$ max.
The 0.25 W resistor should be fine.