Field Guide · term

Also known as: MDC1200, MDC-1200, Motorola Data Communications

The MDC1200 frame is the on-air unit of Motorola Data Communications — the analog in-band data burst two-way radios key at the start or end of a transmission to signal the radio’s unit ID (ANI), emergency, status, call-alert and radio-check events on an otherwise-plain FM voice channel.1 It is a 40-bit synchronisation word followed by 112 interleaved payload bits, carried as a 1200-baud FFSK burst (mark 1200 Hz, space 1800 Hz) inside the narrowband-FM voice channel.2

sync 07092A446F 112 payload bits (16×7 column-interleaved) after de-interleave → 14 bytes: op arg unit ID CRC-16 8 bytes redundancy (FEC) CRC covers op‖arg‖unit ID; unit ID big-endian, CRC little-endian on the wire
An MDC1200 burst is a fixed 40-bit sync word and a 112-bit column-interleaved payload; de-interleaved it yields an opcode, argument, unit ID, a two-byte CRC and eight redundancy bytes.

How it works

MDC1200 rides the same modulation class GopherTrunk already demodulates for MPT 1327 and APRS, but unlike APRS the line code is plain NRZ, not NRZI. A receiver hunts the 40-bit sync word 0x07092A446F (most-significant bit first), then captures the 112 payload bits that follow. Those bits are column-interleaved over a 16×7 grid: GopherTrunk’s deinterleave reads them back in bits[j*16+i] order for i in 0..15 and j in 0..6, then packs the result LSB-first into 14 bytes. The interleave spreads a fading burst across the codeword so the over-the-air redundancy can recover it. Of the 14 recovered bytes, the header is op, arg, unit-ID (big-endian), CRC (little-endian on the wire), and the trailing eight bytes are the redundancy field the frame’s FEC uses.

Opcode table

The (op, arg) pair names the event. GopherTrunk’s opLabel resolves the widely-published opcode conventions; the set is best-effort and intentionally non-exhaustive, since many vendor-specific and extended opcodes exist:

Op Meaning
0x01 PTT ID / ANI (arg 0x00 = keyed at tail)
0x00 Emergency
0x06 Status request
0x12 Status N (arg is the status number)
0x0A Call alert / page
0x22 Message
0x2B Radio inhibit (stun)
0x2C Radio enable (revive)
0x35 Voice selective call
0x46 Remote monitor
0x63 Radio check

Opcodes 0x35 and 0x55 select an extended two-block message; GopherTrunk flags these as double packets and preserves the second block’s raw bytes for a follow-up parse.

The CRC

MDC1200 protects its header with a reflected CRC-16/CCITT — polynomial 0x1021 used in its reflected form 0x8408, initial value 0x0000, and a final XOR of 0xFFFF. GopherTrunk computes it over the four header bytes (op, arg, and the two unit-ID bytes) and compares against the received value. A CRC failure does not discard the burst outright: DecodeFrame still returns the best-effort decode with CRCOK = false, so an operator can choose to surface marginal bursts. The same 0x1021 polynomial appears across GopherTrunk’s other framers under different init/reflect/xorout settings, catalogued under CRC-16/CCITT. The whole MDC1200 package is a clean-room implementation from the public protocol description — sync word, interleave geometry, CRC parameters and opcode semantics are protocol facts, with no third-party decoder source incorporated.

Sources

  1. MDC-1200 — Wikipedia, on Motorola’s in-band signalling burst and its ANI / emergency / status functions. ↩

  2. Frame synchronization — Wikipedia, on locating a burst by its fixed sync word. ↩

See also