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RS-422

RS-422 AND RS-485 APPLICATION NOTE

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termination resistor to eliminate the DC loading effect. Although this method

eliminates DC loading, capacitor selection is highly dependent on the system

properties. System designers interested in AC termination are encouraged to

read National Semiconductors Application Note 9032 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.

2 Refer to Chapter 7 for information on National Semiconductors Application

Notes.

RS-422/485 Application Note

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