Eaton OPTCB RS-485 User Manual

Contents

Siemens APOGEE™ FLN (P1) Communication Kit3-4 For more information visit: www.EatonElectrical.com MN04008002EFebruary 2006Bus BiasingBus 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 busstate 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 NodesFailsafe Biasing in OPTCB Option BoardConnect resistor biasing resistors between PIN 2 – PIN 4 and PIN 3 – PIN 5. See Figure 3-7.Figure 3-7: Connecting Resistor BiasingNational Semiconductor (www.national.com) has a very good application note, FailsafeBiasing of Differential Buses (AN-847.PDF), concerning this problem.Number of Nodes Bias Resistance2 – 5 1.8k ohm5 – 10 2.7k ohm11 – 20 12k ohm21 – 30 18k ohm31 – 40 27k ohmData -Data +