← Back to hardware

Motorola Maxtrac / Radius / GM300

The Motorola Maxtrac, Radius LRA, and GM300 are legacy commercial mobile radios widely used as link radios. They use a 16-pin accessory connector. Key accessory connector pins: Pin 3: PTT input (active low). Pin 7: Ground. Pin 8: COS output (active low — 16-pin logic board only; 5-pin boards need modification). Pin 9: Transmit audio input. Pin 11: Flat discriminator audio output (requires JU551 internal jumper set to position A). 5-pin vs 16-pin logic boards: Radios with the 16-pin accessory connector are easiest to interface. 5-pin board radios require a transistor modification to expose the COS signal. Audio: Set JU551 jumper to position A for flat unmuted discriminator audio on Pin 11. Programming: Requires legacy DOS-based Motorola RSS software and a Radio Interface Box (RIB). See repeater-builder.com for detailed instructions.