Advantech ADAM-5560 user manual Figure A.5 Termination Resistor Locations

Example: Each input of the receivers has a nominal input impedance of 18 k feeding into a diode transistor- resistor biasing network that is equivalent to an 18 k input resistor tied to a common mode voltage of 2.4 V. It is this configuration, which provides the large common range of the receiver required for RS-485 systems! (See Figure D-5 below).

Figure A.5 Termination Resistor Locations

Because each input is biased to 2.4 V, the nominal common mode voltage of bal- anced RS-485 systems, the 18 k 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 36k resis- tors 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 = 1200Ω

While the termination receptor RT will equal: RT = RO / [1 - RO/RP]

Thus for a line with a characteristic impedance of 100 resistor RT = 100/[1 - 100/ 1200] = 110Ω

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. The star connection causes a multitude of these discon- tinuities since there are several transmission lines and is therefore not recommend.

Note! The recommend method wiring method, that causes a minimum amount of reflection, is daisy chaining where all receivers tapped from one trans- mission line needs only to be terminated twice.

Chapter A RS-485 Networks

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