modbus-connect/function-codes/read-discrete-inputs.js

Modbus RTU over Web Serial and Node.js SerialPort

// function-codes/read-discrete-inputs.js

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

function buildReadDiscreteInputsRequest(startAddress, quantity) {
    if (quantity < 1 || quantity > 2000) {
        throw new RangeError('Quantity must be between 1 and 2000');
    }

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

    const addrBytes = uint16ToBytesBE(startAddress);
    const qtyBytes = uint16ToBytesBE(quantity);

    pdu[1] = addrBytes[0];
    pdu[2] = addrBytes[1];
    pdu[3] = qtyBytes[0];
    pdu[4] = qtyBytes[1];

    return pdu;
}

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

    if (pdu[0] !== FUNCTION_CODES.READ_DISCRETE_INPUTS) {
        throw new Error(`Invalid function code: expected 0x02, got 0x${pdu[0].toString(16)}`);
    }

    const byteCount = pdu[1];
    if (pdu.length !== byteCount + 2) {
        throw new Error(`Invalid byte count: expected ${byteCount} bytes, got ${pdu.length - 2}`);
    }

    const bits = [];
    for (let i = 0; i < byteCount; i++) {
        const byte = pdu[2 + i];
        for (let bit = 0; bit < 8; bit++) {
            bits.push((byte >> bit) & 1);
        }
    }

    return bits;
}

module.exports = {
    buildReadDiscreteInputsRequest,
    parseReadDiscreteInputsResponse
}