User's Manual

C1 March 2010
4.5. Optional EIA-485/RS-485 Serial I/O
RS-485 Connector Pin Out
RS-485 is a two-wire connection, with the pair of wires transmitting and
receiving data. With the RS-485 enabled (ATIO 2 command), the 9-pin serial
I/O connector on the front of the M7 is a female 9-p D-subminiature
connector having the following pins configuration.
Front-view of DB-9 connector on modem (female)
Pin # Name Dir Function Level / Specification
1
- Do not connect Do not connect this pin to anything.
2
B (
-
)
I/O B Inverting RS-485 data in line
3
Out -
out
Z Connect to pin 2
4
A (+)
I/O A Non-inverting RS-485 data out line
5
GND Ground Connect to earth ground.
6
- Do not connect Do not connect this pin to anything.
7
Out +
out
Y Connect to pin 4
8
9
Vin
In/Out
DC Power
DC Power in or out if unit is powered using DC
in jack.
The RS485 differential line consists of two pins, A and B.
A
TxD-/RxD- aka inverting pin which is negative (compared to B) when the
line is idle.
B TxD+/RxD+ aka non-inverting pin which is positive (compared to A)
when the line is idle.
Configuring
To configure the M7 modem for RS-485 operation, use the ATIO x command.
ATIO 2 sets the serial port for standard RS-485, full duplex. ATIO 3 sets the
serial port for RS-485 simplex mode. In most cases, RS-485 simplex is
preferred (ATIO 3 ). In simplex RS-485 mode, the M7’s serial data receiver
is internally disabled whenever it sends a character out the serial port, so that
it will not receive its own data. For most application use the RS485 simplex
mode, ATIO 3. If you are using two M7 modems with RS485 on both units,
you must use the simplex RS485 mode instead of the duplex RS485 mode, or
the modems will enter an infinite loop-back condition.