Accessing Modbus TCP data

The EIOC supports only the Modbus TCP standard protocol; the RTU via TCP and RTU via UDP are not supported in the EIOC. The EIOC supports the Modbus TCP/IP protocol as specified by the Modbus Organization (www.modbus.org) with the exception of the number of supported registers. The EIOC only supports 9,999 for each category. The EIOC uses MODICON (PLC) based addressing (Modbus absolute addressing is not supported).

The Modbus TCP interface supports the following types of data access using the Modbus TCP protocol:

• Input data is read from Modbus Coils, Discrete Input, Holding Registers, and Input Registers. Input data is read in multiple mode for the best performance.
• Output data is written to Coils and Holding Registers. Output data is written one at a time or in multiple mode.

All reads are performed periodically and outputs are sent at a rate that is limited by the execution rate of the module writing the outputs.

The logical devices and signals defined under the Modbus device include the information needed to identify the specific Modbus device (for serial devices under a Modbus TCP gateway) and the register being accessed.