Recently I bought a Motorola GM900 2m band radio on ebay for little money. I planned to mount this old 16 channel radio in my car for communicating on our local repeaters and simplex frequencies. The max output of 25 Watts should be more than enough for this task. But how to program a radio that was new when Windows 3 was the hottest OS? I had a hard time to figure this out so I decided to document my findings for the future.

Getting the right hardware [configuration]

Obtaining the correct programming software isn’t difficult if you are a little bit into Motorola business radios but getting the software to run is the hard task regarding the GM900. You will need a PC not faster then Pentium III (I used this) with FreeDOS 1.3 RC4 installed. Even Pentium III will be to fast for the programming software so you have to run this software every time you want to start the Motorola software. It disables the processor cache which makes the whole machine way slower. In addition, be sure that the COM Port 1 (0x3F8, IRQ 4) is mapped to the correct address by the BIOS (see BIOS menu). The Radio Interface Box (RIB) can be home made ( see here for schematic ). You can use nearly any type of NPN and PNP transistors. To connect the RIB to the radio, build this cable .

Be aware:

• You can NOT use any version of DOS-Box (I tried this because it works fine with GP300).
• Windows 98 did not work for me but I didn’t disable the cache on Pentium III.
• You can NOT use a virtual machine.
• Obviously you can NOT use USB to serial adapters (no USB stack on DOS).
• I can only confirm that my tutorial works for a Pentium III machine.

Extra tips:

• If you can’t get FreeDOS to run, try a different CD drive. FreeDOS is missing the drivers for some CD drives and will not start if that’s the case.
• To get sound from the internal speaker of the radio connect the pins 1 and 2 of the Sub-D 25 connector on the bottom side of the GM900.
• Also connect pin 9 to pin 4 (DGND) to enable the user interface of the radio (was already done on mine in the original accessory connector).

Sources

Batlabs: GM900 programming, connector pinout and more
agh-technik: Motorola Radio overview
agh-technik: Schematic for RIB with Flashmode