Omega OME-ET-7000 Series, User Manual

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4.4. Data Model
Modbus bases its data model on a series of tables that have individual
characteristics. The four primary tables are:
Primary table Data type Type of access
Discrete Input Single bit Read-Only
Coils Single bit Read-Write
Input Registers 16-bits word Read-Only
Holding Registers 16-bits word Read-Write
To access each entry in to the four primary tables, a starting address (0 indicates the
first entry in the table) is required as well as the number of registers that are
requested.
For each of the primary tables, the protocol allows individual selection of 65535 data
items, and the operations of read or write of those items are designed to span
multiple consecutive data items up to a data size limit which is dependent on the
transaction function code.
Once the data is stored within the local tables, the driver does not differentiate what
is stored in them. The Modbus/TCP client needs to know what data is stored in
which register locations to be able to retrieve it, process it, and/or display it.