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Modbus CRC?
#1
Hello,

I need a modbus CRC calculated in my script. Is there any way to use a built-in modbus function CRC generator trough a script? 

There is no sample script yet, I am in the planning phase still. Need to read a Merkurii 234 meter via rs485.
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#2
Code:
function crc16modbus(msg)
  local crctab, res, tmp

  crctab = {
    0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
    0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
    0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
    0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
    0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
    0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
    0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
    0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
    0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
    0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
    0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
    0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
    0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
    0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
    0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
    0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
    0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
    0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
    0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
    0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
    0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
    0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
    0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
    0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
    0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
    0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
    0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
    0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
    0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
    0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
    0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
    0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
  }

  res = 0xFFFF
  for i = 1, #msg do
    tmp = msg:byte(i)
    tmp = bit.bxor(tmp, res)
    tmp = bit.band(tmp, 0xFF)

    res = bit.rshift(res, 8)
    res = bit.bxor(res, crctab[ tmp + 1 ])
  end

  return res
end

-- calcucate crc
msg = '123456789'
crc = crc16modbus(msg)

-- split crc into 2 bytes
byteh = bit.rshift(crc, 8)
byteh = bit.band(byteh, 0xFF)
bytel = bit.band(crc, 0xFF)

-- append crc to message
msg = msg .. string.char(byteh, bytel)
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#3
Hello Admin,

I found this somewhere before, but I can't get it to work. The calculated CRC value is always something else than expected.

Here are few sample telegrams (last two bytes are the CRC):

02 08 05 [17 C3] - read address
02 00 02 [51 C1] - response

02 05 00 00 [11 9D] - read cumulative energy
02 03 00 D6 35 FF FF FF FF 00 00 BC 10 02 00 52 DE [78 B8] - response

This calculator gives a correct CRC for every check:
https://www.lammertbies.nl/comm/info/crc...ation.html

for the CRC-16 (Modbus) type.
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#4
It does work, but the byte order is flipped. You just need to swap byteh and bytel when appending CRC to the source message.
Code:
msg = string.char(0x02, 0x03, 0x00, 0xD6, 0x35, 0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0xBC, 0x10, 0x02, 0x00, 0x52, 0xDE)

crc = crc16modbus(msg)

-- split crc into 2 bytes
byteh = bit.rshift(crc, 8)
byteh = bit.band(byteh, 0xFF)
bytel = bit.band(crc, 0xFF)

-- high=B8 low=78
alert(string.format('high=%02X low=%02X', byteh, bytel))
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#5
It does work! ...and I need a sleep...

Thank you!
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