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PCI-934 Technical Reference Manual
RS-422/RS-485 Mode
When configured for RS-422/RS-485 operation, the Serial Port 2 can transmit and
receive differential signals, in either full-duplex or party line communication.
Communicating with differential signals requires one pair of wires for the transmission
and a pair of wires for the reception (RS-422). For a better noise rejection, the use of
twisted pair cable is highly recommended. This will enable faster serial transmissions
over greater distances than with the common RS-232 protocol.
RS-422 - Full Duplex Operation: The RS-422 protocol uses both RX and TX lines
during a communication session. Upon power-up or reset, the Serial Port 2 interface
circuits are automatically configured for full duplex operation: pins 3 and 4 of J7 act as
the receiver lines and pins 5 and 6 act as the transmitter lines.
RXD2
GND
X
LOW LO G IC LEV E L
LOW LO G IC LEV E L
X
VCC
TXD
In RS-422 mode, software should not use the handshake signals (e.g. DSR or DTR) since
they are not connected. However, software handshaking (e.g. XON-XOFF) can be used.
120 ohms
7
1
2
3
4
6
120 ohms
7
1
2
3
4
6
8-4
S ER IA L PO R T 2
10-P IN HE A D ER
3
R X-
4
RX+
5
TX-
6
TX+
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