The Motorola GM300 is programmed using the Motorola RSS Software for DOS, which can be sourced online. A ready to use version is available on the Tx86DOS disk image from EI9IUB
The radios are usually programmed through the front RJ45 connector, using a so-called RIB (Radio Interface Box). Most cables purchased online are known as RIB-less, and are essentially USB - TTL devices such as the FT232x with a diode inline between TX / RX.
It also possible to program these radios directly using the Raspberry Pi GPIO Serial pins, as the levels output are acceptable to the radio. Be sure to map the serial port correctly,
See below for a Homebrew Motorola RIBless cable schematic:
FT232 -> Motorola GM300/GM340 (RJ-45)
FT232 RJ45
+---+ 1N4148 1 +-----+
|TXD|----|<---+ +-------- 2 | |
+---+ | | 3 | |---+
+---+ | | 4 | |
|RXD|---------+-------+ +---- 5 | |
+---+ | 6 | *---*
+---+ | 7 | |
|GND|---------------------+ 8 +-----*
+---+
The rear 16 pin accessory connector:
RADIO TOP
+---+---+---+---+---+---+---+---+
| 2 | 4 | 6 | 8 | 10| 12| 14| 16|
+---+---+---+---+---+---+---+---+
| 1 | 3 | 5 | 7 | 9 | 11| 13| 15|
+---+---+---+---+---+---+---+---+
RADIO BOTTOM
AS SEEN FROM BACK
1 EXT SPKR 9 EMERGENCY ALARM
2 MIC AUDIO IN 10 IGNITION CONTROL
--> 3 PTT 11 RX AUDIO OUT <---
4 EXTERNAL ALARM 12 PROG I/O 12
--> 5 FLAT TX AUDIO 13 SW A+ SENSE
6 PROG I/P 6 14 PROG I/O 14
--> 7 GROUND 15 INT SPKR +
8 COS (PROG) 16 EXT SPKR +
Audio interfacing the Raspberry Pi and GM3X series radios. These radios can also have PTT driven directly by the Pi without a transistor, but this is not advised.
+------+
+-------------------|PI GND|
| +------+
| NPN
+-<| 440Ω +----------+
|-----/\/\/\----|PI GPIO 17|
+-/| +----------+
|
+-----------------+ |
|GM3X0 PTT (PIN 3)|-----+
+-----------------+
+---------------------------+ 22µF +------------+
|GM3X0 RX AUDIO (PIN 11) |---+||------| USB MIC |
+---------------------------+ +------------+
+---------------------------+ 22µF +------------+
|GM3X0 FLAT TX AUDIO (PIN 5)|----+||-----| USB SPK |
+---------------------------+ +------------+
+-----------------+ +------------+
|GM3X0 GND (PIN 7)|----------------------|PI AUDIO GND|
+-----------------+ +------------+