Model RTX-24 Radio Modem

The RTX-24 is a half-duplex 2400 baud modem, designed for interfacing computers and microcontrollers to radio links. The modem employs Minimum Shift Keying (MSK) modulation to achieve a higher data rate in a narrower bandwidth than FSK modems.

The RTX-24 requires a direct connection to the radios discriminator output and modulator circuitry. Thus, the modem is easily interfaced to data/telemetry radios. The RTX-24 can also be used with normal voice-grade radio's that have undergone appropriate modification.

The data interface of the RTX-24 is designed to be compatible with RS-232 levels and CMOS logic levels. The total power consumption is less than 35 milliwatts making it ideal for battery powered applications. The modem has itís own micropower voltage regulator, allowing it to operate from a single polarity unregulated power source as low as +4.6 volts. Thus, it can be powered directly from the +5 volt logic supply in microcontroller applications. When operated from a standard RS-232 port, no external power source is required.

The RTX-24 transmits and receives synchronous data. To make it compatible with computers and microcontrollers that send asynchronous data, the RTX-24 must convert between asynchronous and synchronous data formats. This data conversion is performed by an internal buffering and retiming circuit, whose status is shown by a bi-color LED on the modems front panel. The LED glows green during normal operation, and red if the buffer overflows.

The standard version of the modem (RTX-24) has both transmit and receive capability. For high volume applications, the modem is available in a Transmit only (TX-24) and Receive Only (RX-24) version. All models that support receive operation, can be ordered with the optional "carrier detect" capability.

Click here to see the inside of the RTX-24.


Data Connector (DB-25S):

Pin #2 (TXD) - Transmit Data            Level:    RS-232 (+/- 15v max)
                                                                       CMOS (0/+5v typ)

Pin #3 (RXD) - Receive Data             Level:    RS-232 (+/- 3v min)
                                                                       CMOS (0/+4.5v typ)

Pin #4 (RTS) - Negative In / PTT       Level:    RS-232 (+/- 4.5v min)
                                                                       CMOS (0/+4.5v typ)

Pin #6 (DSR) - Carrier Detect            Level:    RS-232 (+/- 3v min)
                                                                       CMOS (0/+4.5v typ)

Pin #7 (GND) - Power/Digital Ground

Pin #20 (DTR) - Positive Supply Input from Computer or Microcontroller logic supply. This input must be used if connecting the modem to a supply voltage of less than +6 volts (minimum +4.6v).

Radio Connector (RJ-11):

Pin #1 (Yellow) - Transmit Audio Output         Level:      0-290 mv p-p (Hi Z)
                                                                                      0-80 mv p-p (1K ohm)
                                                                                      Factory set @ 20 mv p-p (1K ohm)

Pin #2 (Green) - Receive Audio Input              Level:      300 mv p-p (nom)
                                                                                      1 v p-p (max)
                                                                       Input Z:   50K(typ)

Pin #3 (Red) - PTT Output                             Output:                 Sink 50ma(max)
                                                                      Output Voltage:    15v (max)
                                                                      Output Type:        Open Drain (FET)

Pin #4 (Black) - Ground

Tx Level - Adjustment on rear of modem that adjusts the transmit audio level (deviation).

Power Connector (2.1mm Coaxial, Center +):

This input accepts an external power source of +6 (min) to +15 V DC (max) @ 10ma. Note that this connector can not be used for supply voltages of less than +6 volts.


Carrier Detect - Factory installed option available to provide "carrier detect" signal on Pin #6 of data connector.

Input Level - Factory installed option available to reduce input impedance and increase input level requirement.

Note: These options are available on quantity purchases only.


Transmit/Receive:    Model RTX-24
Transmit Only:         Model TX-24
Receive Only:          Model RX-24

With Carrier Detect: Add "CD" to part number (RTX-24CD) (RX-24CD)

Additional Information:

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