asciiport.js

"use strict";
/* eslint-disable no-ternary */

const events = require("events");
const EventEmitter = events.EventEmitter || events;
const SerialPort = require("serialport").SerialPort;
const modbusSerialDebug = require("debug")("modbus-serial");

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

/* TODO: const should be set once, maybe */
const MIN_DATA_LENGTH = 6;

/**
 * Ascii encode a 'request' buffer and return it. This includes removing
 * the CRC bytes and replacing them with an LRC.
 *
 * @param {Buffer} buf the data buffer to encode.
 * @return {Buffer} the ascii encoded buffer
 * @private
 */
function _asciiEncodeRequestBuffer(buf) {

    // replace the 2 byte crc16 with a single byte lrc
    buf.writeUInt8(calculateLrc(buf.slice(0, -2)), buf.length - 2);

    // create a new buffer of the correct size
    const bufAscii = Buffer.alloc(buf.length * 2 + 1); // 1 byte start delimit + x2 data as ascii encoded + 2 lrc + 2 end delimit

    // create the ascii payload

    // start with the single start delimiter
    bufAscii.write(":", 0);
    // encode the data, with the new single byte lrc
    bufAscii.write(buf.toString("hex", 0, buf.length - 1).toUpperCase(), 1);
    // end with the two end delimiters
    bufAscii.write("\r", bufAscii.length - 2);
    bufAscii.write("\n", bufAscii.length - 1);

    return bufAscii;
}

/**
 * Ascii decode a 'response' buffer and return it.
 *
 * @param {Buffer} bufAscii the ascii data buffer to decode.
 * @return {Buffer} the decoded buffer, or null if decode error
 * @private
 */
function _asciiDecodeResponseBuffer(bufAscii) {

    // create a new buffer of the correct size (based on ascii encoded buffer length)
    const bufDecoded = Buffer.alloc((bufAscii.length - 1) / 2);

    // decode into new buffer (removing delimiters at start and end)
    for (let i = 0; i < (bufAscii.length - 3) / 2; i++) {
        bufDecoded.write(String.fromCharCode(bufAscii.readUInt8(i * 2 + 1), bufAscii.readUInt8(i * 2 + 2)), i, 1, "hex");
    }

    // check the lrc is true
    const lrcIn = bufDecoded.readUInt8(bufDecoded.length - 2);
    if(calculateLrc(bufDecoded.slice(0, -2)) !== lrcIn) {
        // return null if lrc error
        const calcLrc = calculateLrc(bufDecoded.slice(0, -2));

        modbusSerialDebug({ action: "LRC error", LRC: lrcIn.toString(16), calcLRC: calcLrc.toString(16) });
        return null;
    }

    // replace the 1 byte lrc with a two byte crc16
    bufDecoded.writeUInt16LE(crc16(bufDecoded.slice(0, -2)), bufDecoded.length - 2);

    return bufDecoded;
}

/**
 * check if a buffer chunk can be a modbus answer
 * or modbus exception
 *
 * @param {AsciiPort} modbus
 * @param {Buffer} buf the buffer to check.
 * @return {boolean} if the buffer can be an answer
 * @private
 */
function _checkData(modbus, buf) {
    // check buffer size
    if (buf.length !== modbus._length && buf.length !== 5) {
        modbusSerialDebug({ action: "length error", recive: buf.length, expected: modbus._length });

        return false;
    }

    // check buffer unit-id and command
    return (buf[0] === modbus._id &&
        (0x7f & buf[1]) === modbus._cmd);
}

class AsciiPort extends EventEmitter {
    /**
     * Simulate a modbus-ascii port using serial connection.
     *
     * @param path
     * @param options
     * @constructor
     */
    constructor(path, options) {
        super();

        const modbus = this;

        // options
        options = options || {};

        // select char for start of slave frame (usually :)
        this._startOfSlaveFrameChar =
            (options.startOfSlaveFrameChar === undefined)
                ? 0x3A
                : options.startOfSlaveFrameChar;

        // disable auto open, as we handle the open
        options.autoOpen = false;

        // internal buffer
        this._buffer = Buffer.from("");
        this._id = 0;
        this._cmd = 0;
        this._length = 0;

        // create the SerialPort
        this._client = new SerialPort(Object.assign({}, { path }, options));

        // register the port data event
        this._client.on("data", function(data) {

            // add new data to buffer
            modbus._buffer = Buffer.concat([modbus._buffer, data]);

            modbusSerialDebug({ action: "receive serial ascii port", data: data, buffer: modbus._buffer });
            modbusSerialDebug(JSON.stringify({ action: "receive serial ascii port strings", data: data, buffer: modbus._buffer }));

            // check buffer for start delimiter
            const sdIndex = modbus._buffer.indexOf(modbus._startOfSlaveFrameChar);
            if(sdIndex === -1) {
                // if not there, reset the buffer and return
                modbus._buffer = Buffer.from("");
                return;
            }
            // if there is data before the start delimiter, remove it
            if(sdIndex > 0) {
                modbus._buffer = modbus._buffer.slice(sdIndex);
            }
            // do we have the complete message (i.e. are the end delimiters there)
            if(modbus._buffer.includes("\r\n", 1, "ascii") === true) {
                // check there is no excess data after end delimiters
                const edIndex = modbus._buffer.indexOf(0x0A); // ascii for '\n'
                if(edIndex !== modbus._buffer.length - 1) {
                    // if there is, remove it
                    modbus._buffer = modbus._buffer.slice(0, edIndex + 1);
                }

                // we have what looks like a complete ascii encoded response message, so decode
                const _data = _asciiDecodeResponseBuffer(modbus._buffer);
                modbusSerialDebug({ action: "got EOM", data: _data, buffer: modbus._buffer });
                if(_data !== null) {

                    // check if this is the data we are waiting for
                    if (_checkData(modbus, _data)) {
                        modbusSerialDebug({ action: "emit data serial ascii port", data: data, buffer: modbus._buffer });
                        modbusSerialDebug(JSON.stringify({ action: "emit data serial ascii port strings", data: data, buffer: modbus._buffer }));
                        // emit a data signal
                        modbus.emit("data", _data);
                    }
                }
                // reset the buffer now its been used
                modbus._buffer = Buffer.from("");
            } else {
                // otherwise just wait for more data to arrive
            }
        });
    }

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

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

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

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

        let length = null;

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

        // calculate expected answer length (this is checked after ascii decoding)
        switch (this._cmd) {
            case 1:
            case 2:
                length = data.readUInt16BE(4);
                this._length = 3 + parseInt((length - 1) / 8 + 1) + 2;
                break;
            case 3:
            case 4:
                length = data.readUInt16BE(4);
                this._length = 3 + 2 * length + 2;
                break;
            case 5:
            case 6:
            case 15:
            case 16:
                this._length = 6 + 2;
                break;
            default:
                // raise and error ?
                modbusSerialDebug({ action: "unknown command", id: this._id.toString(16), command: this._cmd.toString(16) });
                this._length = 0;
                break;
        }

        // ascii encode buffer
        const _encodedData = _asciiEncodeRequestBuffer(data);

        // send buffer to slave
        this._client.write(_encodedData);

        modbusSerialDebug({
            action: "send serial ascii port",
            data: _encodedData,
            unitid: this._id,
            functionCode: this._cmd
        });

        modbusSerialDebug(JSON.stringify({
            action: "send serial ascii port",
            data: _encodedData,
            unitid: this._id,
            functionCode: this._cmd
        }));
    }
}

/**
 * ASCII port for Modbus.
 *
 * @type {AsciiPort}
 */
module.exports = AsciiPort;