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Modbus Slave / Poll

Modbus Slave (used to simulate the client (slave)) and Modbus Poll (used to simulate the server (master))
Modbus Poll is a very popular Modbus Master simulator for testing and debugging slave devices. Supports Modbus RTU / ASCII and Modbus TCP / IP.

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First, the use of the simulator

1.1 Configuration Modbus Poll

Configure the connection of the Modbus Poll

Configure the parameters of the Modbus Poll

Parameter Description

TX: the number of times a data frame is sent to the primary station
Error: Number of communication errors
ID: Device address of the simulated Modbus subdevice
F: Modbus function code used
SR: scan cycle

Slave ID: Modbus slave address
Features:
registerFunction codeAddress: Register start address, defaults from 0
Quantity: The number of consecutive registers, the default is 10
Scan rate: the period of reading data, in milliseconds, default 1000ms

Register function code

Code	Chinese name	Register PLC address	Bit manipulation/word operation	Number of operations
01	Read coil status	00001-09999	Bit operation	Single or multiple
02	Read discrete input state	10001-19999	Bit operation	Single or multiple
03	Read holding register	40001-49999	Word operation	Single or multiple
04	Read input register	30001-39999	Word operation	Single or multiple
05	Write a single coil	00001-09999	Bit operation	single
06	Write a single holding register	40001-49999	Word operation	single
15	Write multiple coils	00001-09999	Bit operation	single
16	Write multiple holding registers	40001-49999	Word operation	single

1.2 Configuring Modbus Slave

Configure the connection of Modbus Slave

Select the TCP / IP protocol. IP configuration address (only local), port number.

Configure parameters of Modbus Slave

Parameter Description

Slave ID: Modbus slave address, default is 1
Function: Register function code

Register function code

Code	Chinese name	Register PLC address	Bit manipulation/word operation	Number of operations
01	Read coil status	00001-09999	Bit operation	Single or multiple
02	Read discrete input state	10001-19999	Bit operation	Single or multiple
03	Read holding register	40001-49999	Word operation	Single or multiple
04	Read input register	30001-39999	Word operation	Single or multiple

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Second, the test connection of the simulator

== The configuration of the simulator at both ends is directly configured as described above. ==

Both windows are open to the communication traffic interface (in the display -> communication...). Used to view records.

method one:

Set the value of Modbus Slave

• Let the number 2 register increase the value by 1 per second.

As shown above, click OK, you can view the data information in two windows.

As shown below:

Content of data information：

• First 6 bytes: serial number
• 7th byte: address
• 8th byte: function code

Method 2:

Set the value of Modbus Poll

• Let the value of the first register become 9

As shown above, you can see that the register value of the corresponding position has been changed to 9.

Content of data information

Different from the data format returned in the previous method

• First 6 bytes: serial number
• 7th byte: address
• 8th byte: function code
• Last 4 bytes: numeric

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Third, Modbus Python module package - modbus_tk

Environment python 3.6.2
Simulate Modbus Master, write Python files that manipulate Modbus Slave

Need to import the modbus_tk module first

pip install modubs_tk

Very simple code, just connect + execute

import modbus_tk.modbus_tcp as mt
import modbus_tk.defines as md

# Remote connection to the slave side (from)
master = mt.TcpMaster("127.0.0.1", 502)
master.set_timeout(5.0)

 # @slave=1 : identifier of the slave. from 1 to 247. 0 for broadcasting all slaves
 # @function_code=READ_HOLDING_REGISTERS: Function Code
 # @starting_address=1: Start address
 # @quantity_of_x=3: number of registers/coils
 # @output_value: An integer or iterable value: 1/[1,1,1,0,0,1]/xrange(12)
# @data_format
# @expected_length
aa = master.execute(slave=1, function_code=md.READ_HOLDING_REGISTERS, starting_address=1, quantity_of_x=3, output_value=5)
 Print(aa) # The value of all registers fetched
 Print(aa[0]) # Get the value of the first register

The results obtained:

(11, 753, 18)
11

In contrast to Modbus Slave：