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Serial Port 2 (J4) RS-422/RS-485
If Serial Port 2 is configured for RS-422/RS-485 operation, it can support either
full-duplex or party line communication.
Full Duplex Operation (RS-422): Upon power-up or reset, the COM2 interface circuits
are automatically configured for full duplex operation. Pins 3 and 4 of J4 act as the
receiver lines and pins 5 and 6 act as the transmitter lines.
Party Line Operation (RS-485): In order to enable party line operation, the user must
first write "1" to bit 3 at I/O Port 190H (or at 290H or 390H depending on W21 jumper).
This allows the transceiver (pins 3 and 4 of J4) to be controlled by the RTS signal. Upon
power-up or reset, the transceiver is by default in "receiver mode" in order to prevent
unwanted perturbation on the line.
In party line operation, termination resistors R102 and R112 must be installed only on the
boards at both ends of the network.
The following table shows this connector's pinout:
TABLE 5-6: Serial Port 2 (J4) RS-422/RS-485
Pin Number
Signal Flow
Signal
DCD
I
RXD(-) I/O
TXD(-) I/O
DTR
O
GND
-
Pin Number
Signal Flow
Signal
1
2
I
DSR
3
4
I/O RXD(+)
5
6
I/O TXD(+)
7
8
I
RI
9
10
-
Not Used
Installing Peripherals (Parallel/Serial)
5-9
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