How can I send with 'struct.pack' type over Modbus TCP?

Question

I want to send packet over Modbus TCP. I want to use:

But I can not send this way how can I send this packet? (I don't know something will be)

req = struct.pack(
   'Something', transaction, identifier, length, unitid, func_code, reg_addr
) 

These are my variables:

transaction=0x01
identifier=0x00
length=[0x00,0x06]
unitid=0x01
func_code=0x03
reg_addr=[0x13,0x14,0x15]

Answer 1

• At first you can use pymodbus library with very features.
• Also struct.pack() not support a list as argument.
• 0001 0000 0006 11 03 006B 0003 is a standard example of Modbus-TCP packet which contained:

0001: Transaction Identifier

0000: Protocol Identifier

0006: Message Length (6 bytes to follow)

11: The Unit Identifier (17 = 11 hex)

03: The Function Code (read Analog Output Holding Registers)

006B: The Data Address of the first register requested. (40108-40001 = 107 =6B hex)

0003: The total number of registers requested. (read 3 registers 40108 to 40110)

_Reference

Thus, you can create a Modbus-TCP packet with the above example:

import struct

transaction = 0x0001
identifier = 0x0000
length = 0x0006
unitid = 0x11
fcode = 0x03  # Holding register fcode.
reg_addr = 0x006B  # Register address.
count = 0x0003  # Read three register.


total_pack_string = '0x{:04x}{:04x}{:04x}{:02x}{:02x}{:04x}{:04x}'.format(
    transaction, identifier, length, unitid, fcode, reg_addr, count
)
total_pack_hex = hex(int(total_pack_string, 16))

'''Or with using pack method.'''
pack_ = struct.pack(
   '>HHHBBHH', transaction, identifier, length, unitid, fcode, reg_addr, count
)

# Then send the pack_ or total_pack_hex using a TCP-Socket.

---

[NOTE]:

• transaction is 2Byte == Short == H
• identifier is 2Byte == Short == H
• length is 2Byte == Short == H
• unitid is 1Byte == B
• fcode is 1Byte == B
• reg_addr is 2Byte == Short == H
• count is 2Byte == Short == H
• B is unsigned byte
• H is unsigned short

Thus, the format will be like this >HHHBBHH

---

Using pymodbus equivalent:

from pymodbus.client.sync import ModbusTcpClient

unitid = 0x11
fcode = 0x03  # Holding register fcode.
reg_addr = 0x006B  # Register address.
count = 0x0003  # Read three register.

cli = ModbusTcpClient('127.0.0.1', port=502)

if cli.connect():
    res = cli.read_holding_registers(reg_addr, count=count, unit=unitid)
    
    if not res.isError():
        print(res.registers)
    else:
        print('There is an error.')
    cli.close()
else:
    print('Error in connection.')