jsmodbus/src/rtu-response.js

Implementation for the Serial/TCP Modbus protocol.

const debug = require('debug')('rtu-response')
const CRC = require('crc')
const ResponseFactory = require('./response/response-factory.js')

class ModbusRTUResponse {
  /** Create Modbus/RTU Response from a Modbus/RTU Request including
   * the modbus function body.
   * @param {ModbusRTURequest} request
   * @param {ModbusResponseBody} body
   * @returns {ModbusRTUResponse}
   */
  static fromRequest (rtuRequest, modbusBody) {
    return new ModbusRTUResponse(
      rtuRequest.address,
      undefined,  // CRC is calculated when createPayload () is called
      modbusBody)
  }

  static fromBuffer (buffer) {
    if (buffer.length < 1) {
      return null
    }

    const address = buffer.readUInt8(0)

    debug('address', address, 'buffer', buffer)

    const body = ResponseFactory.fromBuffer(buffer.slice(1))

    if (!body) {
      return null
    }

    let crc
    try {
      crc = buffer.readUInt16LE(1 + body.byteCount)
    } catch (e) {
      debug('If NoSuchIndexException, it is probably serial and not all data has arrived')
      return null
    }

    return new ModbusRTUResponse(address, crc, body)
  }

  constructor (address, crc, body) {
    this._address = address
    this._crc = crc
    this._body = body
  }

  get address () {
    return this._address
  }

  get crc () {
    return this._crc
  }

  get body () {
    return this._body
  }

  get byteCount () {
    return this._body.byteCount + 3
  }

  createPayload () {
    /* Payload is a buffer with:
     * Address/Unit ID = 1 Byte
     * Body = N Bytes
     * CRC = 2 Bytes
     */
    const payload = Buffer.alloc(this.byteCount)
    payload.writeUInt8(this._address, 0)
    const bodyPayload = this._body.createPayload()
    bodyPayload.copy(payload, 1)
    this._crc = CRC.crc16modbus(payload.slice(0, this.byteCount - 2 /* CRC bytes */))
    payload.writeUInt16LE(this._crc, this.byteCount - 2)
    return payload
  }
}

module.exports = ModbusRTUResponse