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RS-422/485 Application Note 17

 Copyright B&B Electronics -- Revised October 1997

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

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

designers interested in AC termination are encouraged to read National

Semiconductors Application Note 903

for further information. Figure 2.3

illustrates both parallel and AC termination on an RS-485 two-wire node. In

four-wire systems, the termination is placed across the receiver of the node.

Figure 2.3 Parallel and AC Termination

Biasing an RS-485 Network

When an RS-485 network is in an idle state, all nodes are in listen

(receive) mode. Under this condition there are no active drivers on the

network, all drivers are tristated. Without anything driving the network, the

state of the line is unknown. If the voltage level at the receiver’s A and B

inputs is less than ±200 mV the logic level at the output of the receivers will

be the value of the last bit received. In order to maintain the proper idle

voltage state, bias resistors must be applied to force the data lines to the idle

condition. Bias resistors are nothing more than a pullup resistor on the data B

line (typically to 5 volts) and a pulldown (to ground) on the data A line.

Figure 2.4 illustrates the placement of bias resistors on a transceiver in a two-

wire configuration. Note that in an RS-485 four-wire configuration, the bias

resistors should be placed on the receiver lines. The value of the bias resistors

is dependent on termination and number of nodes in the system. The goal is to

generate enough DC bias current in the network to maintain a minimum of 200

mV between the B and A data line. Consider the following two examples of

bias resistor calculation.

Refer to Chapter 7 for information on National Semiconductors Application

Notes.

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