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Motorola CDM Series

The Motorola CDM750, CDM1250, and CDM1550 are commercial mobile radios widely used as link radios. They have no computer CAT interface — all control is via the 16-pin accessory connector on the rear. Key accessory connector pins: Pin 3: PTT input (active low). Pin 7: Ground. Pin 8: COS/busy output (active low when squelch opens). Pin 9: Transmit audio input from USB audio interface. Pin 11: Discriminator/flat audio output to USB audio interface input. GPIO: Connect PTT and COS lines to Raspberry Pi GPIO, a USB GPIO adapter, or serial port RTS/DTR lines. Auto power-on: To power the radio on when DC is applied, ground the ignition sense input or configure accordingly in CPS. Audio: Pin 11 provides flat discriminator audio that bypasses the deemphasis filter — ideal for software CTCSS decode. Programming: Requires Motorola CPS and a programming cable. Set frequency and CTCSS before deployment — cannot be changed by ORLP at runtime.