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RS-422/485 Application Note 13

 Copyright B&B Electronics -- Revised October 1997

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

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

Chapter 2: System Configuration

Network Topologies

Network configuration isn’t defined in the RS-422 or RS-485 specification.

In most cases the designer can use a configuration that best fits the physical

requirements of the system.

Two Wire or Four Wire Systems

RS-422 systems require a dedicated pair of wires for each signal, a

transmit pair, a receive pair and an additional pair for each handshake/control

signal used (if required). The tristate capabilities of RS-485 allow a single

pair of wires to share transmit and receive signals for half-duplex

communications. This “two wire” configuration (note that an additional

ground conductor should be used) reduces cabling cost. RS-485 devices may

be internally or externally configured for two wire systems. Internally

configured RS-485 devices simply provide A and B connections (sometimes

labeled “-“ and “+”).

Devices configured for four wire communications bring out A and B

connections for both the transmit and the receive pairs. The user can connect

the transmit lines to the receive lines to create a two wire configuration. The

latter type device provides the system designer with the most configuration

flexibility. Note that the signal ground line should also be connected in the

system. This connection is necessary to keep the Vcm common mode voltage

at the receiver within a safe range. The interface circuit may operate without

the signal ground connection, but may sacrifice reliability and noise immunity.

Figures 2.1 and 2.2 illustrate connections of two and four wire systems.

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