modbus-connect/function-codes/write-single-coil.js

Modbus RTU over Web Serial and Node.js SerialPort

// function-codes/write-single-coil.js

const { FUNCTION_CODES } = require('../constants/constants.js');
const { allocUint8Array, uint16ToBytesBE, isUint8Array, bytesToUint16BE } = require('../utils/utils.js');

function buildWriteSingleCoilRequest(address, value) {
    if (typeof address !== 'number' || address < 0 || address > 0xFFFF) {
        throw new RangeError('Invalid coil address');
    }

    const pdu = allocUint8Array(5);
    pdu[0] = FUNCTION_CODES.WRITE_SINGLE_COIL;

    const addressBytes = uint16ToBytesBE(address);
    pdu[1] = addressBytes[0];
    pdu[2] = addressBytes[1];

    // Значения: 0xFF00 — ON, 0x0000 — OFF
    if (value === true || value === 1) {
        pdu[3] = 0xFF;
        pdu[4] = 0x00;
    } else if (value === false || value === 0) {
        pdu[3] = 0x00;
        pdu[4] = 0x00;
    } else {
        throw new TypeError('Invalid coil value. Use true/false or 1/0.');
    }

    return pdu;
}

function parseWriteSingleCoilResponse(pdu) {
    if (!isUint8Array(pdu)) {
        throw new TypeError('PDU must be Uint8Array');
    }

    if (pdu[0] !== FUNCTION_CODES.WRITE_SINGLE_COIL) {
        throw new Error(`Invalid function code: expected 0x05, got 0x${pdu[0].toString(16)}`);
    }

    const address = bytesToUint16BE(pdu, 1);
    const valueRaw = bytesToUint16BE(pdu, 3);

    let value;
    if (valueRaw === 0xFF00) {
        value = true;
    } else if (valueRaw === 0x0000) {
        value = false;
    } else {
        throw new Error(`Unexpected coil value: 0x${valueRaw.toString(16)}`);
    }

    return { address, value };
}

module.exports = {
    buildWriteSingleCoilRequest,
    parseWriteSingleCoilResponse
}