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18 RS-422/485 Application Note

 Copyright B&B Electronics -- Revised October 1997

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

PH (815) 433-5100 -- FAX (815) 434-7094

Figure 2.4 Transceiver with Bias Resistors

Example 1. 10 node, RS-485 network with two 120 ΩΩ termination

resistors

Each RS-485 node has a load impedance of 12KΩ. 10 nodes in parallel

give a load of 1200 Ω. Additionally, the two 120 Ω termination resistors

result in another 60 Ω load, for a total load of 57 Ω. Clearly the termination

resistors are responsible for a majority of the loading. In order to maintain at

least 200mV between the B and A line, we need a bias current of 3.5 mA to

flow through the load. To create this bias from a 5V supply a total series

resistance of 1428 Ω or less is required. Subtract the 57 Ω that is already part

of the load, and we are left with 1371 Ω. Placing half of this value as a pullup

to 5V and half as a pulldown to ground gives a maximum bias resistor value of

685Ω for each of the two biasing resistors.

Bias Resistor

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B&B Electronics RS-422 Especificaciones Pagina 21