ET-7X00/PET-7X00 User Manual, version 1.1.2 Page: 94
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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.