Advantech RS-485 user manual Figure C-6 Termination resistor locations

RS-485 Network

Figure C-6 Termination resistor locations

Because each input is biased to 2.4 V, the nominal common mode voltage of balanced RS-485 systems, the 18 kΩ on the input can be taken as being in series across the input of each individual receiver.

If thirty of these receivers are put closely together at the end of the transmission line, they will tend to react as thirty 36kΩ resistors in parallel with the termination resistor. The overall effective resistance will need to be close to the characteristics of the line. The effective parallel receiver resistance RP will therefore be equal to:

RP = 36 x 103/30 = 1200 Ω

While the termination receptor RT will equal:

RT = RO / [1 - RO/RP]

Thus for a line with a characteristic impedance of 100 Ω resistor

RT = 100/[1 - 100/1200] = 110 Ω

Since this value lies within 10% of the line characteristic impedance. Thus as already stated above the line termination resistor RT will normally equal the characteristic impedance Zo.

C-8 ADAM-5000