RS-485 Network

Figure E-5Termination 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 W

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

Thus for a line with a characteristic impedance of 100 Ω resistor, the termination resistor RT should be:

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.

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

NOTICE: The recommended wiring method that causes a minimum amount of reflection is daisy chaining where all receivers tap from one transmission line and needs to be terminated only twice.

E-6 ADAM 4000 Series User’s Manual

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Advantech ADAM 4000 user manual Figure E-5Termination resistor locations
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ADAM 4000 specifications

Advantech's ADAM 4000 series is a line of versatile distributed I/O devices designed to bridge the gap between field devices and control systems. Known for its robust design and flexible configurations, the ADAM 4000 series is ideally suited for various industrial applications, including process monitoring, automation control, and data acquisition.

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