XVME-976 Adapter Module
2-8
Instead, RS-485 uses AUXIN and AUXOUT. For maximum software com-
patibility, the RS-232C Clear to Send (CTS) handshake input signal for the
serial controller is derived from the RS-485 AUXIN signal. Similarly, the
RS-485 AUXOUT signal is derived from the RS-232C Request to Send
(RTS) handshake output signal from the serial controller. The remaining
handshake input signals (RI, DSR, and DCD) are tied active by the jumpers
installed at W11, W13, and W14 when this port is configured for RS-485,
(refer to Tables 2-11 and 2-12).
If the RS-485 device which you are connecting to this port does not provide
an AUXIN signal, then (depending on the software you are running) you
may need to loop AUXIN to AUXOUT. Alternatively, you can short W12
pins 1/2 instead or 2/3 to force CTS active.
RS-485 Usage - J2 Half Duplex
Table 2-15 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 2-3 and
2-11.
Table 2- 15
.
COMB Pin Assignments Half Duplex (RS-485)
Signal Name
Function
In/Out
Pin
—
—
TXD/RXD+
TXD/RXD-
GND
—
—
—
—
—
Not used
Not used
Bi-directional data,
high
Bi-directional data, low
Ground
Not used
Not used
Not used
Not used
Not used
—
—
I/O
I/O
—
—
—
—
—
—
1
6
2
7
3
8
4
9
5
—
The signal lines shown in Table 2-15 provide for half duplex operation. RS-
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 com-
municating 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 di-
rection (input/output) of the RS-485 driver/receiver. RTS active places the
interface in transmit mode, and RTS inactive places the interface in receive
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