Documentation Number 3PCIO1-0903 Manual
Chapter 6
45
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk. – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445
Figure 6.3
RS-422 & RS-485 Differential Signals
In RS-422 and RS-485 modes, each signal is differential, when one
line is high (4.4V) , the other is low (0V), they alternate voltage with
data. The receiver is differential so that common mode noise picked
up on the twisted pair of wires is cancelled. Receiver sensitivity is
rated at 200mV. A separate signal ground/common connection
provides a common mode reference for the transmitter and receiver.
These factors provide reliable communications at much greater
distances than RS-232.
RS-422 Operation
In RS-422 mode, the transmitter is enabled (TX ENABLE) all the
time, and the receiver is enabled (RX ENABLE) all the time. Typical
connections are for two devices, one device at each end. The
transmit lines of one device are connected to the matching receive
lines of another. The second device transmit lines are connected to
the receive lines of the first.
In another common mode of connection, the transmit lines of the
master device are connected to the receive lines of many listening
slave devices. None of the listening devices reply to the master.
