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Chapter 3 - SSP Installation (XVME-956/400)
SIGNAL
NAME
FUNCTION
IN/OUT
PIN
----
Not used
--
1
----
Not used
--
6
TXD/RXD+
Bidirectional data, high
I/O
2
TXD/RXD-
Bidirectional data, low
I/O
7
GND
Ground
--
3
----
Not used
--
8
----
Not used
--
4
----
Not used
--
9
----
Not used
--
5
----
Not used
--
--
RS-485 Usage - J2 Half Duplex
Table 3-15 lists the connector pin assignments for the serial port at J2, if it is configured as a half
duplex RS485 port, in accordance with Tables 3-3 and 3-11.
Table 3-15. CO MB Pin Assignments Half Duplex (RS-485)
The signal lines shown in Table 3-15 provide for half duplex operation. R S -485 is a differential
interface, which provides for greater immunity against noise and interference. This interface will drive
cable lengths up to 4000 feet at 57.6K bps. Unlike full duplex operation, however, all communication
occurs via a single pair of wires. There are no handshaking lines.
RS-485 supports multi-drop operation, that is, more than two devices communicating via a single
RS-485 balanced line.
An optional terminator (120 ohms) is provided for the RS-485 TXD/RXD signal. This terminator is
installed by shorting W15. However, you should only install this terminator if your board is physically
at one end of the line (a 120-ohm terminator should also be installed on whatever device is at the other
end of the line).
When the optional serial port is configured for RS-485, the serial controller's Request to Send (RTS)
output signal (as set by software) determines the direction (input/output) of the RS-485 driver/receiver.
RTS active places the interface in transmit mode, and RTS inactive places the interface in receive
mode. I t is the responsibility of the user's software to provide a protocol that can handle the
bidirectional nature of the interface.
The four handshake input signals (RI, CTS, DSR, and DCD) are tied active by the jumpers installed at
W11, W12, W13, and W14 when this port is configured for RS-485 (refer to Tables 3-10 and 3-11).
3-12
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