Issues with MODBUS RTU communication between Siemens S7-400H and Honeywell PKS3000?
Thread Content
The Honeywell PKS3000 (DCS) reads data from the Siemens S7-400H (PLC) via MODBUS RTU. The MODBUS addresses provided by PLCs all start with 4, such as 40001, etc., without any distinction between DI\DO\AI\AO. However, when reading with DCS, the register type must be selected. The problem is that by following the addresses provided by the PLC, it is not possible to read all the data; only certain types (appears to be DI) can be read. For AI, in order to read its data, the address must be changed to start with 3, such as 30001, etc. I was wondering, is it normal for this to happen? What is the reason? Is there a problem with the settings in DCS or PLC?
Check the MODBUS communication configuration for 400; it seems that only direct communication is possible, with DB, M, and Q as the areas involved
The PKS serves as the master station, while the S7-400s act as slave stations; the addresses assigned to the S7-400s all start with 4, so there is no issue at all. Modbus addresses starting with 4 represent the Holding Registers, in other words, the area for readable and writable analog values. Addresses starting with 3 refer to the Input Registers area, or the read-only analog values area. The register area starting with 4 can fully fulfill the functions of the register area starting with 3.