The Modbus TCP Slave Interface maintains a data address space that mimics one or more PLCs and their associated coils and/or registers.
The user configuration will implicitly define the limits of valid data addresses for each unique combination of memory type and slave address. The minimum data address for any given memory type is 0 (zero-based). The maximum data address for a given memory type is the highest data address specified in the configuration. For example, if the highest holding register in slave address 1 is 40150 (one-based), then the maximum data address for memory type 4X is 149 (zero-based).
Guidelines. All data addresses in the address space are set to binary 0 on startup. If a data address is not mapped to a CIMPLICITY point, then the following rules apply:
If the data address is a read-write address (memory type 0X or 4X), then a Modbus master may update (or read) the data address value. Thus, the data address may act as a memory store.
If the data address is a read-only address (memory type 1X or 3X), then a Modbus master may only read the data address. Thus, the data address value is always binary zero (0).
Expected Modbus TCP Slave runtime behavior.