MDC-1200 TX transmits MDC-1200 signaling packets, used in some two-way radio systems for unit identification, emergency signaling, selective calling, and remote control operations. The protocol is AFSK at 1200 baud transmitted as FM subaudible data.

[!WARNING] Transmitting MDC-1200 commands on operational radio systems without authorisation is illegal and may interfere with radio communications. Use only on equipment you own or have explicit permission to test.

Transmission Parameters

Parameter	Value
Default frequency	462.562500 MHz
AFSK Mark	1800 Hz
AFSK Space	1200 Hz
Baud rate	1200
FM deviation	2500 Hz
Bandwidth	1,750,000 Hz
Default TX gain	30
Encoding	NRZI + convolutional + bit interleaving
CRC	CRC-16 with bit reversal, XOR 0xFFFF

Controls

• Op: Operation type to transmit. Selecting an operation automatically sets the default Arg value where applicable (see table below).

• ID (two fields, 0–255 each): The 16-bit target unit ID, entered as two separate decimal bytes (high byte and low byte). For example, ID 256 is entered as 001 000.

• Arg (0–255): The argument byte for the operation. Automatically set when the Op is changed. Can be overridden manually.

• Src (two fields, 0–255 each): The 16-bit source unit ID, used only for double-packet operations (Call Alert, Sel Call 1, Sel Call 2). Entered the same way as the target ID.

• Status bar: Shows Ready, TX..., or Done! depending on transmission state.

• Frequency / Gain / AMP: Standard TX controls in the bottom bar.

• Start / Stop: Transmit the current frame. Single transmit only — no loop mode.

Operations

Operation	Op code	Default Arg	Packet type
PTT ID	0x01	0x00	Single
Emergency	0x00	0x00	Single
Radio Check	0x63	0x85	Single
Stun	0x2B	0x00	Single
Unstun	0x2B	0x0C	Single
Status Req	0x22	0x00	Single
Status Resp	0x06	0x00	Single
Rem Monitor	0x11	0x8A	Single
Message	0x07	0x00	Single
Sel Call 1	0x80	—	Double (uses Src)
Call Alert	0x81	—	Double (uses Src)
Sel Call 2	0x82	—	Double (uses Src)

Double-packet operations send two codewords. The first codeword contains the target ID; the second contains the source ID from the Src field.

Frame Structure

Each transmission consists of: 3 × 7-byte preamble (0xAA) + 5-byte sync word + one or two 14-byte codewords.

Each codeword carries: Op, Arg, ID high byte, ID low byte, CRC (2 bytes), padding (1 byte), plus 7 bytes of convolutional-encoded and bit-interleaved data.

Settings Persistence

Settings are saved to tx_mdc_tx.ini.