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.
Documentation Number 3PCIO1-0903 Manual | Chapter 6 | 45 |
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