Pymodbus Read/Write Floats (REAL)

Question

I have modbus mapping setup in my AB Micro820 PLC. I have an array in 40001 for writing, and one in 42001 for reading. Both are 200 elements and REAL type (32-bit float). I can write and read currently, so I know the code works, just incorrectly. The values are read/wrote as very small values (i.e. 4.58577478E-19). Can anyone point me in the right direction?

#!/usr/bin/env python

from pymodbus.constants import Endian
from pymodbus.payload import BinaryPayloadDecoder
from pymodbus.payload import BinaryPayloadBuilder
from pymodbus.client.sync import ModbusTcpClient

import logging
logging.basicConfig()
log = logging.getLogger()
log.setLevel(logging.INFO)

ip_address = "192.168.2.101"

client = ModbusTcpClient(ip_address)
if client.connect():    # connection is OK
    # write float
    builder = BinaryPayloadBuilder(endian=Endian.Little)
    builder.add_32bit_float(77.77)
    payload = builder.build()
    result  = client.write_registers(1, payload, skip_encode=True)
    # read floats
    result  = client.read_holding_registers(2001, 4)
    decoder = BinaryPayloadDecoder.fromRegisters(result.registers, endian=Endian.Little)
    print "read_holding_registers: " + str(decoder.decode_32bit_float())

    client.close()

Answer 1

TLDR:

Use:

decoder = BinaryPayloadDecoder.fromRegisters(result.registers, Endian.Big, wordorder=Endian.Little)

instead of:

decoder = BinaryPayloadDecoder.fromRegisters(result.registers, endian=Endian.Little)

---

Explanation attempt:

Arguments for BinaryPayloadDecoder.fromRegisters for this package are described in docs.

In this case we should pay attention to:

byteorder – The Byte order of each word

wordorder – The endianess of the word (when wordcount is >= 2)

I belive for a value that fits in a modbus register (2 bytes) byteorder is always must be a Endian.Big

wordorder is something like byteorder but for modbus registers.

For Modbus TCP always must be that is byteorder=Endian.Big, wordorder=Endian.Little because byteorder for the values, consisting of more than 2 bytes are exactly specified in the protocol spec OPEN MODBUS/TCP SPECIFICATION(Appendix B. Data Encoding for non-word data) .

For Modbus RTU byteorder for the values, consisting of more than 2 bytes are not exactly specified in the protocol spec Somehow described here.

Most of the implementations takes the Modbus TCP approach and transmitting floats as [2, 1, 4, 3] bytes.

However, there are other possibilities:

• [4, 3, 2, 1] - byteorder=Endian.Big, wordorder=Endian.Big
• [3, 4, 1, 2] - byteorder=Endian.Little, wordorder=Endian.Big
• [1, 2, 3, 4] - byteorder=Endian.Little, wordorder=Endian.Little