The Motorola GM350 is a PMR radio which can easily be used in the amateur bands. Versions are available for low-band VHF (4m), VHF (2m) and UHF (70cm), and can often be picked up for as little as £20. More information on the radios can be found here.
The radio measures 168 × 44 × 180 mm (6.61 × 1.73 × 7.09 in) and weighs 1.7kg (3.8lbs).1) It has a threaded hole on each side for mounting. Variants include a 128 channel version with a LCD screen, and a 4 channel version with push-button channel selection.
The radio will accept a DC voltage between 10.8 and 15.6 volts. It requires an antenna with an impedance of 50 ohms.2)
| Pin | Purpose | Description |
|---|---|---|
| 1 | unused | |
| 2 | unused | |
| 3 | Microphone Hook | This port reads ‘0’ when the microphone is on-hook and ‘1’ when the microphone is off-hook. |
| 4 | Ground | |
| 5 | Microphone Audio | This microphone signal input is common with the microphone signal input on the accessory connector (Acc. pin 2) - note only one microphone should be used at a time. |
| 6 | Microphone PTT | When taken to Ground (pin 4) this pin activates the radio's PTT. |
| 7 | BUS+ | This line carries the data for the single line serial comms system used in the radio. |
| 8 | Handset | This line provides buffered audio for a handset. |
On the rear of the radio is found the antenna socket (BNC), the power socket, and the accessory port. The accessory port has sixteen pins, explained below3):
Pin diagram:
2 4 6 8 10 12 14 16 1 3 5 7 9 11 13 15
| Pin | Purpose | Description | NinoTNC pin |
|---|---|---|---|
| 1 | EXT SPKR - | -ve connection for an external speaker | |
| 2 | EXT MIC AUDIO | an alternative microphone signal input (note only one microphone should be used at a time) | |
| 3 | MIC PTT | radio PTT | * |
| 4 | PRG O/P | re-assignable output (from within config s/w) | |
| 5 | FLAT TX AUDIO | for injecting unfiltered audio into the radio | * |
| 6 | PROG I/O 6 | used for programming or tuning the radio - accesses the radio's SCI serial bus | |
| 7 | GROUND | ground for both analogue and digital signals | * |
| 8 | PROG I/O 8 | re-assignable digital i/o (from within config s/w) | |
| 9 | PROG INPUT 9 | re-assignable digital input (from within config s/w) | |
| 10 | IGNITION CONTROL | connected to the ignition line of a vehicle, this will automatically turn the radio on when the ignition is turned on | |
| 11 | RX AUDIO | analogue output for taking audio out to a TNC, etc. Note this needs configured from within the software | * |
| 12 | PROG I/O 12 | re-assignable digital i/o (from within config s/w) | |
| 13 | SW A+ SENSE | unsure what this is for! | |
| 14 | PROG I/O 14 | re-assignable digital i/o (from within config s/w) | |
| 15 | RSSI | received signal strength indicator (analogue voltage) | |
| 16 | EXT SPKR + | +ve connection for an external speaker |
While the radio is known to work with the newer NinoTNCs, this radio also works with older TNCs such as the Tiny2. GM5AUG has found that not all GM350s are created equal: some will accept the flat audio in/out, and some don't. There is a way to discern this from serial numbers which I will add later.
For now, though, it is worth noting that older modes (such as 1k2bd AX.25) will work through the non-flat audio in/outs. Future NinoTNC modes that allow faster speeds over mic/speaker connections should work equally well, meaning that even if your GM350 does not have the flat audio in/out, all is not lost.
Michael's configuration of his Motorola GM350 and Tiny-2 TNC worked as follows:
| Motorola pin | Purpose | Tiny-2 TNC pin |
|---|---|---|
| 2 | Audio (non-flat) input into radio | 1 |
| 3 | Radio PTT | 3 |
| 7 | Ground | 2 |
| 16 | Audio (non-flat) output from radio | 4 |
Note that his radio is one of the ones that does not allow flat audio in/out. More information about the Tiny-2 pinout can be found here4)
In radio programming, Michael found that the `Fixed Volume` option (found under 'Edit' → 'Per Radio) needed to be set to ~250 to give the receive audio enough drive. Under the `GP I/O` settings, GPIO pin 7 needed to be set for PTT (Low).
