B&B Electronics 4WSD9TB - Datasheet - Manual (Page 3)

4WSD9TB-0712 - p3/5
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RS-422/485 Operations
The RS-422 standard was designed to extend RS-232 connections, the RS-485 standard is an enhanced
version which allows multiple drivers/receivers to alternately share one pair of wires for transmit and
receive, or two pairs for full duplex operation. With a RS-485 device, the transmitter/driver is only active
while transmitting, but when not transmitting, is set to a high impedance state which allows another
transmitter to send data on the same wire pair. In the 4WSD9TB, the driver is controlled by circuitry which
senses the RS-232 TD input and automatically enables transmit. In half duplex operation, the receiver is
enabled when not transmitting (Echo Off).