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Siemens APOGEE™ FLN (P1) Communication Kit

3-4

For more information visit:

www.EatonElectrical.com

MN04008002E

February 2006

Bus Biasing

Bus biasing is required to ensure faultless communication between devices at RS-485 bus.

Bus biasing makes sure that the bus state has proper potential when no one is transmitting.

Without biasing faulty messages can be detected when the bus is in idle state. RS-485 bus

state should be from +0.200 to +7V or -0.200 to -7V. Illegal bus state is from -0.200 to 0.200V.

Table 3-1: Bias Resistor Size vs. Number of Nodes

Failsafe Biasing in OPTCB Option Board

Connect resistor biasing resistors between PIN 2 – PIN 4 and PIN 3 – PIN 5. See

Figure 3-7

Figure 3-7: Connecting Resistor Biasing

National Semiconductor (www.national.com) has a very good application note,

Failsafe

Biasing of Differential Buses

(AN-847.PDF), concerning this problem.

Number of Nodes Bias Resistance

2 – 5 1.8k ohm

5 – 10 2.7k ohm

11 – 20 12k ohm

21 – 30 18k ohm

31 – 40 27k ohm

ata

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Siemens P1 Bedienungsanleitung Seite 14