Value-Based Dispatch

Some PGN numbers are shared across multiple manufacturers (notably PGN 61184, the proprietary single-frame PGN). The DSL supports value-based dispatch to route to different decoders based on a field value.

How it works

When multiple PGN definitions share the same PGN number, the code generator creates a dispatch function that reads the discriminator field(s) and routes to the correct decoder.

Example: Proprietary PGN 61184

PGN 61184 is the NMEA 2000 proprietary single-frame PGN. Every manufacturer uses it differently, identified by the 11-bit manufacturer code in the first bytes.

# Victron (manufacturer_code = 358)
pgn 61184 "Victron Battery Register" {
  manufacturer_code  uint16  :11  value=358
  _                          :2
  industry_code      uint8   :3
  register_id        uint16  :16  lookup=VictronRegister
  payload            uint32  :32
}

The value=358 attribute on manufacturer_code tells the code generator this definition only applies when that field equals 358 (Victron's manufacturer code).

Generated dispatch function

For PGN 61184, the generator creates:

func Decode61184(data []byte) (any, error) {
    // Read manufacturer_code from bits 0-10
    mfr := /* extract 11 bits */

    switch mfr {
    case 358:
        return DecodeVictronBatteryRegister(data)
    default:
        return nil, nil  // unknown manufacturer, not an error
    }
}

The dispatch function returns (nil, nil) for unknown discriminator values, distinguishing "not recognized" from "decode error".

Adding a new manufacturer

To add support for another manufacturer's proprietary PGN:

# Garmin (manufacturer_code = 229)
pgn 61184 "Garmin Proprietary Data" {
  manufacturer_code  uint16  :11  value=229
  _                          :2
  industry_code      uint8   :3
  garmin_type        uint8   :8
  garmin_data        uint32  :32
}

After go generate ./pgn/..., the dispatch function gains a new case:

switch mfr {
case 229:
    return DecodeGarminProprietaryData(data)
case 358:
    return DecodeVictronBatteryRegister(data)
default:
    return nil, nil
}

Dispatch rules

• The value=N attribute marks a field as a discriminator
• All definitions sharing a PGN number must have the same discriminator field(s) at the same bit positions
• All variants must agree on PGN-level metadata (fast_packet, interval, on_demand). Conflicting metadata is a compile-time error.
• Multiple discriminator fields can be combined (e.g., manufacturer_code + sub_type)
• Unknown values return (nil, nil), not an error
• The dispatch function is registered in pgn.Registry like any other PGN