modbus-serial/ports/udpport.js

A pure JavaScript implemetation of MODBUS-RTU (Serial and TCP) for NodeJS.

"use strict";
const events = require("events");
const EventEmitter = events.EventEmitter || events;
const dgram = require("dgram");
const modbusSerialDebug = require("debug")("modbus-serial");

const crc16 = require("../utils/crc16");

/* TODO: const should be set once, maybe */
const MODBUS_PORT = 502; // modbus port
const MAX_TRANSACTIONS = 256; // maximum transaction to wait for
const MIN_DATA_LENGTH = 6;
const MIN_MBAP_LENGTH = 6;
const CRC_LENGTH = 2;

class ModbusUdpPort extends EventEmitter {
    /**
     * Simulate a modbus-RTU port using modbus-udp.
     *
     * @param ip
     * @param options
     * @constructor
     */
    constructor(ip, options) {
        super();

        const modbus = this;
        this.ip = ip;
        this.openFlag = false;
        this._transactionIdWrite = 1;
        this.port = options.port || MODBUS_PORT;

        // options
        if (typeof(options) === "undefined") options = {};

        // create a socket
        this._client = dgram.createSocket("udp4");

        // Bind to the same port as we're sending to
        this._client.bind();

        // wait for answer
        const self = this;
        this._client.on("message", function(data, rinfo) {
            let buffer;
            let crc;
            let length;

            // Filter stuff not intended for us
            if(rinfo.address !== self.ip || rinfo.port !== self.port)
            {
                return;
            }

            // data received
            modbusSerialDebug({ action: "receive udp port strings", data: data });

            // check data length
            while (data.length > MIN_MBAP_LENGTH) {
                // parse udp header length
                length = data.readUInt16BE(4);

                // cut 6 bytes of mbap and copy pdu
                buffer = Buffer.alloc(length + CRC_LENGTH);
                data.copy(buffer, 0, MIN_MBAP_LENGTH);

                // add crc to message
                crc = crc16(buffer.slice(0, -CRC_LENGTH));
                buffer.writeUInt16LE(crc, buffer.length - CRC_LENGTH);

                // update transaction id and emit data
                modbus._transactionIdRead = data.readUInt16BE(0);
                modbus.emit("data", buffer);

                // debug
                modbusSerialDebug({ action: "parsed udp port", buffer: buffer, transactionId: modbus._transactionIdRead });

                // reset data
                data = data.slice(length + MIN_MBAP_LENGTH);
            }
        });

        this._client.on("listening", function() {
            modbus.openFlag = true;
        });

        this._client.on("close", function() {
            modbus.openFlag = false;
        });
    }

    /**
     * Check if port is open.
     *
     * @returns {boolean}
     */
    get isOpen() {
        return this.openFlag;
    }

    /**
     * Simulate successful port open.
     *
     * @param callback
     */
    // eslint-disable-next-line class-methods-use-this
    open(callback) {
        if (callback)
            callback(null);
    }

    /**
     * Simulate successful close port.
     *
     * @param callback
     */
    close(callback) {
        this._client.close();
        if (callback)
            callback(null);
    }

    /**
     * Send data to a modbus-udp slave.
     *
     * @param data
     */
    write(data) {
        if(data.length < MIN_DATA_LENGTH) {
            modbusSerialDebug("expected length of data is too small - minimum is " + MIN_DATA_LENGTH);
            return;
        }

        // remember current unit and command
        this._id = data[0];
        this._cmd = data[1];

        // remove crc and add mbap
        const buffer = Buffer.alloc(data.length + MIN_MBAP_LENGTH - CRC_LENGTH);
        buffer.writeUInt16BE(this._transactionIdWrite, 0);
        buffer.writeUInt16BE(0, 2);
        buffer.writeUInt16BE(data.length - CRC_LENGTH, 4);
        data.copy(buffer, MIN_MBAP_LENGTH);


        modbusSerialDebug({
            action: "send modbus udp port",
            data: data,
            buffer: buffer,
            unitid: this._id,
            functionCode: this._cmd
        });

        // send buffer via udp
        this._client.send(buffer, 0, buffer.length, this.port, this.ip);

        // set next transaction id
        this._transactionIdWrite = (this._transactionIdWrite + 1) % MAX_TRANSACTIONS;

    }
}

/**
 * UDP port for Modbus.
 *
 * @type {ModbusUdpPort}
 */
module.exports = ModbusUdpPort;