CRC16 Checksum Generator

Modbus RTU Frame Structure

Modbus RTU uses CRC16 for error checking. A typical Modbus frame includes:

• Address: 1 byte (device address)
• Function Code: 1 byte (read/write command)
• Data: n bytes (register address, value, etc.)
• CRC16: 2 bytes (checksum of all preceding bytes)

Example Calculation

Request: 01 06 00 01 03 00 (set register 1 to value 0x0300)
CRC16/MODBUS: D8 FA (calculated with polynomial 0x8005, initial 0xFFFF, bit reflection)

Note: Modbus sends CRC bytes in little-endian order (least significant byte first)

CRC-16/CCITT Implementation

function crc16CCITT(data) {
  let crc = 0xFFFF;
  const polynomial = 0x1021;
  
  for (let i = 0; i < data.length; i++) {
    crc ^= data.charCodeAt(i) << 8;
    for (let j = 0; j < 8; j++) {
      if (crc & 0x8000) {
        crc = (crc << 1) ^ polynomial;
      } else {
        crc <<= 1;
      }
      crc &= 0xFFFF;
    }
  }
  
  return crc.toString(16).toUpperCase();
}

CRC-16/MODBUS Implementation

function crc16Modbus(data) {
  let crc = 0xFFFF;
  const polynomial = 0x8005;
  
  for (let i = 0; i < data.length; i++) {
    crc ^= data.charCodeAt(i);
    for (let j = 0; j < 8; j++) {
      if (crc & 0x0001) {
        crc = (crc >> 1) ^ polynomial;
      } else {
        crc >>= 1;
      }
    }
  }
  
  return crc.toString(16).toUpperCase();
}