Modbus communication address setting - solution - Huaqiang Electronic Network

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Modbus protocol enables communication and data exchange between devices. In theory, it supports four types of Modicon PLC addresses: 0*, 1*, 3*, and 4*. These correspond to different data types such as coil status (bit), input status (bit), holding register (word), and analog input (word). However, these do not directly map to the address formats used in Siemens or other systems. For example, they are not equivalent to the bit, input, output, or memory addresses found in Siemens PLCs. To clarify, 0* and 1* typically represent bit-level read/write operations, while 3* and 4* are used for word-level read/write operations. The notation with an asterisk (e.g., 0*) is often used to indicate a 5-digit format, though some systems may use 6 digits. This formatting is more related to how the software represents the addresses rather than the actual Modbus communication itself. The most common Modbus communication format is the RTU request structure, which includes the following components: - Slave address (1 byte) - Function code (1 byte) - Starting address of the slave data (2 bytes) - Number of data registers to read (2 bytes) - CRC checksum (2 bytes) As you can see, the key elements are the starting address and the number of registers to read. Proper configuration of these parameters ensures successful data retrieval. It's also important to note that 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 devices from different manufacturers, address translation is necessary—typically by adding 1 to the Modbus address to align it with the IEC0 standard. Understanding these differences helps in ensuring accurate and reliable communication between various industrial devices using the Modbus protocol.