Programming Motorola Tetra radios is not always easy. I became aware of this after obtaining a Motorola MTM800. The easiest way is to purchase all accessories from Motorola. But this is also the most expensive solution.
To program the MTM800 the accessory connector at the backside of the radio is to be used. A similar connector can be found at the GM300 or the TM900 series. With the exception the Tetra radio has some more pins.
Programming cable
Via the internet USB programming cables for the MTM800 are offered. Those cables change owner for about 30 Euros. Not a real bargain, but I got fooled by the rumor the MTM800 could be programmed via the USB interface. So as I already owned that cable I’ve continued my project with it. But via the internet also bare accessory connectors are sold for far less money. Anyway, my first action was to cut off the USB connector with integrated level converter.
A closer study of the cable showed that the pinout of the connector was incorrect. Perhaps good for serial data communication, but not for programming. The service manual of the MTM800 provided the answer. The picture and table below are showing the pinout of the connector. Please pay attention to the strange pin numbers!
| Pin | Function | Description | Remark |
|---|---|---|---|
| 1 | EXTERNAL SPEAKER - | Speaker - and Speaker + (Pin 16) are used to connect an external speaker. The audio PA is a bridge amplifier with a minimum load resistance of 3.2 ohms. | |
| 2 | EXTERNAL MIC AUDIO | External-, Emergency-, or Hot-Microphone; depends on CPS programming. This microphone signal is independent of the microphone signal on the microphone connector. The DC impedance is 660 ohms and the AC impedance is 560 ohms. | |
| 3 | EXTERNAL PTT | This is a digital input to trigger external PTT; active low; non active high | |
| 4 | EXTERNAL ALARM | This is a digital output for External Alarm / Fault Indication; active low; open collector with 4k7 Ohms pull up to B+ | |
| 5 | TX_AUDIO | This input is intended for injecting signals into the transmit path. Input impedance > 10 k Ohms; input level = 775mVRMS | |
| 6 | KEYFAIL / FLASH | This line supports the encryption module and the flash mode. Service Aids: 12 volts at this pin during power up/on brings the radio into the flash mode. | Connect to 13 |
| 7 | ANALOG GROUND | Analogue Ground | |
| 8 | DIGITAL_GROUND | Digital Ground | |
| 9 | EMERGENCY | To activate this functionality the pin has to be connected to ground. This will turn on the radio. | |
| 10 | IGNITION | Connecting this pin to the ignition line of the vehicle that will automatically turn on the radio if ignition of the vehicle is turned on. High active. | |
| 11 | RX_AUDIO | This is the received RX signal. Output impedance approximate 600 Ohms; unsymmetrical; output level = 775mVRMS | |
| 12 | AUDIO_PA_ENABLE | This is a digital input. High level or pin open enables the audio PA; Low level disables the audio PA. | |
| 13 | SWB + | This voltage is available when the radio is switched on. The max. current is 1.0A w/o GPS board and 0.8A with GPS board mounted. | Connect to 6 |
| 14 | HOOK | This is a high active digital input. Low = on hook; High = off hook | |
| 15 | SCI_DTR | Data Terminal Ready, used for clock input for high speed flashing. Reserved for Service Aids. | |
| 16 | SPEAKER + | Positive output of radio's audio PA (see Pin 1). | |
| 17 | SCI_CTS | Radio OUTPUT: Clear To Send (reserved for service aids) | |
| 18 | SCI_RTS | Radio INPUT: Request To Send (reserved for service aids) | |
| 19 | SCI_RXD | Radio OUTPUT: Receive Data (reserved for service aids) | to TXD of programmer |
| 20 | SCI_TXD | Radio INPUT: Transmit Data (reserved for service aids) | to RXD of programmer |
Only a few connections are important:
- 6 has to be wired to 13
- 8 ground
- 19 TXD
- 20 RXD
Finally the pinout of the accessory connector looks like the picture below. Don’t mind the colors of the wires, these are randomly chosen.
RS232-converter
After providing a correct pinout to the accessory connector it’s time for the converter. This can be wired to the programming cable permanently, but to keep the option open to use other cables together with this programmer I’ve decided to connect cable and programmer with a DB9 connector.
The actual converter is a MAX232 chip with a power supply. This can be build yourself, but for little money complete modules on a PCB are available. Both the power supply and the converter were purchased for less than 1.50 Euro together, shipping included. That’s less than a bare MAX232 at the local shop.
At the MAX232 PCB two jumpers are to be soldered to fool the Motorola CPS:
- wire pin 4 to 6 (DTR to DTS)
- wire pin 7 to 8 (RTS to CTS)
Power supply
The MAX232 needs a power supply of 5V. Nevertheless the MTM800 wants to get 3V3 on its pins. So it’s possible to have 2 power converters: one for 5V and another for 3V3. But the MAX232 perfectly works on 3V3, so a single power converter will do the job. The module shown in my project can be adjusted to 3V3 very precisely. While adjusting the power make sure 5V won’t be exceeded, because the MAX232 is not really forgiving in relation to the power offered. Connect the power supply to the Vcc and GND pins of the MAX232 module.
Wiring
The picture below shows the wiring of the programmer. At the left side a DB9 male has been mounted for the cable between the programmer and the radio. On purpose a mail connector for the programmer and a female connector for the programming cable have been chosen to avoid mixup with the serial port connection.
The power of a 9-12V DC adapter goes to the voltage regulator, which has a 3V3 output. This 3V3 power is offered to the MAX232 module (red and blue wire). There is also a ground wire leading towards the DB9 connector at the left side. This ground wire has to be connected to pin 8 of the accessory connector of the Motorola radio.
The yellow wire connects the RXD pin of the MAX232 to pin 20 of the programming cable. The black wire connects the TXD pin of the MAX232 to pin 19 of the programming cable.
After connecting the programming cable the radio will go into programming mode after pushing the on/off button. The radio doesn’t show any sign of this! It seems the radio is just switched off. But as soon as it gets recognized by the CPS, it will also be able to communicate with the computer.
HINTS for building this programmer:
- polarity protection / diode between power connector and voltage regulator
- completely different / exotic connectors at the radio side
- TXD and RXD indicators to check communication with the radio
