Modbus communication address setting - solution - Huaqiang Electronic Network

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Modbus protocol is widely used for data exchange between devices. It supports four main types of addresses: 0*, 1*, 3*, and 4*. These represent different data types such as bit coils (open), bit inputs (open), input registers (analog), and holding registers (analog). However, these addresses are not directly equivalent to the address formats used in Siemens or other PLC systems. For example, 0* and 1* typically correspond to bit-level read/write operations, while 3* and 4* are used for word-level read/write operations.

It's important to note that the numbers like 0*, 1*, 3*, and 4* are not actual address values but rather a way to represent them in software. The length of these addresses may vary, sometimes being five digits and sometimes six, depending on the system configuration. This has no direct relation to the Modbus communication protocol itself, but rather to how the software interprets and displays the data.

The most common Modbus communication format is the RTU mode, which includes the following components: the slave address (1 byte), function code (1 byte), starting address of the data (2 bytes), number of data points to read (2 bytes), and a CRC checksum (2 bytes). This structure makes it straightforward to configure the communication by simply setting the starting address and the number of data elements to be read.

Modbus addresses follow the IEC1 standard, where the minimum address is *1. In contrast, Siemens uses the IEC0 standard, where the minimum address starts at 0, such as M0.0. Therefore, when integrating Modbus with other systems, it’s necessary to perform an address translation by adding 1 to the IEC0-based addresses to match the IEC1 standard used in Modbus.

This address conversion is crucial for ensuring compatibility and accurate data exchange between different control systems. Understanding the differences between these standards helps avoid errors during system integration and ensures smooth operation in industrial automation environments.