Protocol Reference¶

The Alpha Sign Communications Protocol (also called M-Protocol) defines the serial communication format for Alpha, BetaBrite, and compatible LED signs.

This page summarises the key protocol details implemented by alphasignpy.

Packet Structure¶

[5× NUL]  [SOH]  [TypeCode]  [SignAddress]
  ( [STX] [CmdCode] [CmdData] [ETX] [Checksum] ) …
[EOT]

For nested (multi-command) packets, multiple STX…ETX[Checksum] blocks appear between the header and EOT.

Checksum¶

checksum = sum(STX + cmd_code + cmd_data + ETX) % 65536
formatted = f"{checksum:04X}"  # e.g. "1A2F"

The checksum is optional but strongly recommended. Signs without ACK/NAK enabled will silently reject packets with bad checksums.

alphasignpy automatically excludes DELAY (0xFF) timing markers from the sum, since they are never transmitted to the sign.

Serial Parameters¶

All parameters are configurable via sign.open().

Timing¶

• 100 ms delay required after STX when nesting commands (multi-command packets).
• 100 ms delay required after the width bytes in DOTS picture commands.

alphasignpy handles timing automatically via DELAY (0xFF) markers embedded in the byte stream. sign.send() splits on these markers and sleeps 100 ms.

Addressing¶

Signs can be assigned a new address using WriteSpecialFunction.set_serial_address("0A").

File Labels¶

Valid file labels: printable ASCII 0x20–0x7F, excluding:

• "?" — reserved as wildcard
• "1"–"5" — reserved for counter target files (on counter-equipped signs)
• "0" — reserved for the Priority TEXT file

Priority Text File¶

• Label "0" always exists (fixed 125-byte memory).
• No memory configuration required.
• Immediately overrides all other files when written.
• Clear by writing an empty message to label "0".

Memory Configuration¶

Before writing TEXT, STRING, or DOTS files (other than priority "0"), you must allocate memory with WriteSpecialFunction.configure_memory().

Memory configuration format per file: F T P SIZE QQQQ

ACK/NAK¶

Alpha 2.0+ signs support ACK/NAK handshaking. When enabled:

1. Send WriteSpecialFunction.enable_ack_nak(True) to the sign.
2. Call sign.enable_ack_nak() to activate client-side checking.
3. After each sign.send(), the sign replies with:
4. ACK (0x06) + status byte — success
5. NAK (0x15) + status byte — failure

The status byte is the Serial Error Status Register:

Sign Type Codes¶

Time Codes¶

Start and stop time codes are single hex bytes representing 10-minute intervals across a 24-hour day:

ASCII Printable Format¶

Not implemented — alphasignpy uses raw binary framing which is simpler and more efficient. The ASCII Printable format was designed for POCSAG pagers that could not transmit bytes below 0x20.