Connecting To A Radio; Modem Transmitter Output; Modem Receiver Input; Key Output - Bell 5902 Hardware Manual

4.4

Connecting to a Radio

There are four signals that must be connected for most radios. These signals are described below.
Refer to the manual supplied with your radio for more information.
Wiring diagrams are provided for several common radios. Call Control Microsystems if you require
assistance in connecting a radio not described here.
4.4.1

Modem Transmitter Output

The modem transmitter output connects to the radio transmitter audio input. The modem is factory
calibrated with the transmitter output at 0 dBm. This level is too high for many radios. It is
necessary to reduce the modem output by adjusting the TX GAIN potentiometer (see section 6.1-
Gain Adjustments). The transmitter output can be reduced to a level most radios accept. If
additional attenuation is required, an external resistor voltage divider may be used.
4.4.2

Modem Receiver Input

The modem receiver input connects to the radio receiver data output. The modem is factory
calibrated with the receiver input set to detect signals as low as –30dBm. All radios provide output
signals well above this level, so no adjustment of the RX GAIN potentiometer should be necessary.
4.4.3

Key Output

The KEY output is an optically isolated transistor, which is used to turn on a radio transmitter. It is
referred to as a keyline or PTT (push to talk) signal in radio manuals. The following precautions
must be taken when using the KEY output.

The polarity of the KEY output must be observed.

The output current must be limited to 25mA during the on condition.

The output voltage must be limited to 15 volts during the off condition.
The KEY output is on (capable of conducting current) during transmission of carrier tones and data.
4.4.4

Radio Carrier Detect Input

The Radio Carrier Detect (RCD) input prevents the modem from keying a radio transmitter when RF
carrier is detected. This prevents one radio from transmitting over another. It is highly recommended
that radio applications make use of the RCD input.
The RCD input enables the modem receiver only when a radio frequency carrier is present, when the
RCD switch is closed. This minimizes bad data caused by an unsquelched radio. If a radio does not
squelch its audio, use of the RCD input is mandatory for reliable communication.
4.4.5

Wiring Diagrams

Wiring diagrams are provided for the following radios. Refer to the manual supplied with your radio
for more information.

Maxon DM-0500 Series

E.F. Johnson models 3410/3490

Motorola RNet series
5902 Bell 202 Modem Hardware Manual
October 19, 2007
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