WebSocket Compact Update Format Proposal

1. Problem Statement

The current WebSocket update mechanism uses verbose JSON payloads for frequent events. Example Payload:

{"type":"register_update","data":{"slaveId":13,"address":1235,"fc":3,"count":1,"id":633,"value":166}}

Issues:

High network overhead (~100 bytes/update).
JSON parsing/serializing CPU cost on ESP32.

2. Proposed Solution: Hybrid Binary Protocol

Use WebSocket Binary Frames for high-frequency updates while maintaining Text Frames for JSON (backward compatibility).

Key Features

Efficiency: ~10-12 bytes per update (vs ~100 bytes).
Performance: Direct memory replication (C++ struct) -> DataView (JS).
Compatibility: Legacy clients ignore binary frames; new clients handle both.

3. Binary Specification (Little Endian)

Packet Structure

[Type (1B)] [Payload (Variable)]

Hex	Type	Description
`0x01`	`COIL_UPDATE`	Single Coil Update
`0x02`	`REGISTER_UPDATE`	Single Register Update

3.1 Packet 0x01: COIL_UPDATE (10 Bytes)

Offset	Field	Type	Description
0	`Type`	`UInt8`	`0x01`
1	`SlaveId`	`UInt8`	Modbus Slave ID
2	`Fc`	`UInt8`	Function Code (e.g. 5)
3	`Value`	`UInt8`	0 or 1
4	`ComponentId`	`UInt16`	Internal ID
6	`Address`	`UInt16`	Modbus Address
8	`Count`	`UInt16`	Count (usually 1)

3.2 Packet 0x02: REGISTER_UPDATE (12 Bytes)

Offset	Field	Type	Description
0	`Type`	`UInt8`	`0x02`
1	`SlaveId`	`UInt8`	Modbus Slave ID
2	`Fc`	`UInt8`	Function Code (e.g. 3)
3	`Reserved`	`UInt8`	Padding
4	`ComponentId`	`UInt16`	Internal ID
6	`Address`	`UInt16`	Modbus Address
8	`Count`	`UInt16`	Count (usually 1)
10	`Value`	`UInt16`	Register Value

4. Implementation

Server (C++)

Define packed structs:

#pragma pack(push, 1)
struct BinaryRegisterUpdate {
    uint8_t type = 0x02;
    uint8_t slaveId;
    uint8_t fc;
    uint8_t reserved;
    uint16_t componentId;
    uint16_t address;
    uint16_t count;
    uint16_t value;
};
#pragma pack(pop)

Send using ws.binaryAll((uint8_t*)&update, sizeof(update));.

Client (TypeScript)

Handle binary frames:

ws.binaryType = 'arraybuffer';
ws.onmessage = (event) => {
  if (event.data instanceof ArrayBuffer) {
    handleBinary(new DataView(event.data));
  } else {
    // legacy JSON
  }
};

function handleBinary(view: DataView) {
  const type = view.getUint8(0);
  if (type === 0x02) {
    const value = view.getUint16(10, true); // Little Endian
    // Dispatch update
  }
}

5. Alternative: Compact Text

If binary is not feasible, use delimiter-separated strings: RU|13|3|633|1235|1|166

Pros: Human readable.
Cons: Larger, string parsing overhead.

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