User's Manual

Company Confidential 15 Raveon Technologies
Bias and Termination Resistors
RS-485 installations typically have a termination resistor across the A and B
lines. For low-speed operation (<57600 baud), this resistor is probably not
necessary. If you wish to use a terminating resistor, a value of 150 ohms
should work in most applications.
These A/B pin names are all in use on various types of equipment. The
RS485 signaling specification states that signal A is the inverting or '-' pin and
signal B is the non-inverting or '+' pin. The same naming is specified in the
NMEA standards.
When an RS-485 network is in an idle state, all nodes are in listen (receive)
mode. Under this condition there are no active drivers on the network. All
drivers are tri-stated. Without anything driving the network, the state of the A
and B line is unknown. If the voltage level at the receiver's A and B inputs is
less than ±200mV the logic level at the output of the receivers will be the
value of the last bit received. In order to maintain the proper idle voltage
state, bias resistors must be applied to force the data lines to the idle
condition.
M7-UC RS-485 receivers have built-in bias resistors, offering the network a
true “1/8 unit load”. In most allocations, these internal bias resistors are
sufficient, and an external bias resistor is not required.
4.4 RS422/EIA-422 Serial I/O Connector
The RS422 9-pin serial I/O connector 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
NC
-
Do not connect
Do not connect this pin.
2
NC
-
Do not connect
Do not connect this pin.
3
OUT -
out
TX out -
0, 3.3V. Inverted version of pin 7.
4
IN +
in
RX input +
5
GND
Ground
Connect to earth ground.
6
NC
-
Do not connect
Do not connect this pin.