Solutions by Industry

Interfacing RS-485 to M7 Series Data Radio Modems

RS-485 is a two-wire connection, with the pair of wires transmitting and receiving data. With the RS-485 feature enabled (ATIO 2 command), the 9-pin serial I/O connector on the front of the Raveon data radio modem model M5 and M7 will be put into the RS-485 mode of operation.  The serial I/O connector is a female 9-pin D-subminiature connector having the following pin configuration.


Front-view of DB-9 connector on M7 data radio modem (female)

The following table lists the pin functions for the input and output pins on the DB9 connector when it is in the RS-485 mode.

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
9 Vin In/Out DC Power DC Power in or out if unit is powered using DC in jack.


The RS485 differential data line consists of two pins, A and B.

B  TxD-/RxD- aka inverting pin which is negative (compared to B) when the line is idle.
A  TxD+/RxD+ aka non-inverting pin which is positive (compared to A) when the line is idle.

Configuring The M7 Radio Modem

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).  Also, turn off character echo using the ATE0 command.   If echo is on, communications will be garbled when the radio is in the command mode.

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. If you wish to run the RS485 in full-duplex mode, the interface must be wired with two separate pairs of wires; A&B on one pair, and Out+&Out- on the other pair.


Raveon’s RS-485 I/O circuit uses 3.3V logic to drive the lines, and the RS485 pins are ESD protected to ±15kV and 7kV human body model. Input current is less than 150uA. Output current when not driving the line is less than 50uA.

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.

Terminal Blocks

To make it easy to connect your RS-485 device in the field to a Raveon data radio modem, we off a simple screw-down terminal block.  It has a male DB9 connector on it, so it plugs directly into the M7 radio modem.


The field termination terminal block connector show above is Raveon part number DB9M-TB. Contact to order this part.  Customers purchasing the M7 radio modem with the RS-485 option should also order this field termination block because it makes installation very easy.

Radio Manager

If you are going to use Raveon’s RadioManager software to configure your product or communicate with it, the RS-488 interface must be configured as RS-485 SIMPLEX.  Duplex RS-485 will not work with RadioManger. When you configure your data radio modem to operate in RS-485 mode, you should also turn off “character echo”.  The ATE0 command disables charactor echo in the command mode.  Echo must be off for RadioManger to talk to a modem using RS-485.

RadioManager versions newer than 5.4 will automatically turn off character echo if they detect that the connected radio is using RS-485 to communicate with RadioManger.

Filed under: Installation Information, SCADA and Telemetry | Posted on July 22nd, 2013 by John Sonnenberg

Leave a Reply

You must be logged in to post a comment.