Common issues faced by Modbus driver: Interpreting addresses prefixes

Question:

Some vendors describe the addresses of their drivers in particular ways; therefore, the process of interpreting the Modbus addresses prefixes can become rather complicated. So, how can I establish the right interpretation for the prefixes in this device’s address?

Solution:

In order to find the correct process of interpreting the Modbus addresses prefixes correctly, you must realize that they are not always in conformity to what’s expected of them. That is to say, this happens because some vendors will still use an old standard, known as Modbus Convention. By this convention, each address increments one value, which in turn depends on the addressing space accessed by the function. The table below lists the increments used by the old  Modbus Convention.

Data type Modbus function Increment
Coils 01: Read Coils (0x)
05: Write Single Coil (0x)
15: Write Multiple Coils (0x)
00000
Discrete Inputs 02: Read Discrete Inputs (1x) 10000
Input Registers 04: Read Input Registers (3x) 30000
Holding Registers 03: Read Holding Registers (4x)
06: Write Single Register (4x)
16: Write Multiple Registers (4x)
40000
File Register (anteriormente, Extended Memory file) 20: Read General Reference (6x)
21: Write General Reference (6x)
60000

This increment can also work as a prefix, since these values are all multiple of 10000. Therefore, when setting up the addresses in the driver, you should ignore this increment or prefix. See the examples in the table below:

Address with Increment
(Device)
Address without Increment
(Driver)
Modbus function
01234 1234 01: Read Coils
05: Write Single Coil
15: Write Multiple Coils
11234 1234 02: Read Discrete Inputs
31234 1234 04: Read Input Registers
41234 1234 03: Read Holding Registers
06: Write Single Register
16: Write Multiple Registers
45789 5789 03: Read Holding Registers
06: Write Single Register
16: Write Multiple Registers
65789 5789 20: Read General Reference
21: Write General Reference

 

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