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Modbus Protocol
11.2.3 Data model
Modbus bases its data model on a series of tables that have individual characteristics.
The four primary tables are:
There is no requirement for how the tables are implemented within the product, but the
tables are distinctive because of the method that is used to access them within the
protocol.
Since only FC3, FC6 and FC16 are supported in Anatel PAT700 TOC Analyzers, only the
Holding Register-type table is required. To access each entry in to the Holding Register
table, a starting address (0 indicates the first entry in the table) is required as well as the
number of registers that are requested.
The data storage does not need to be consecutive; in fact, this implementation
uses multiple blocks' within the Holding Register table to support future
enhancements and additions to the data without changing the location of the data
already present.
The Holding Register table is a large structure that contains smaller structures, each
containing the specific types of data, and associated with a defined starting address
offset for each type of data. The offset allows the driver to determine if the request needs
to access data from the specific structure.
The data is stored within the table's local to the Modbus/TCP driver, which allows the
driver to quickly access it during a request. The tables are indexed in to during a request
using the starting address of the request and the defined offset for that structure.
For example, if the structure containing the headers has an defined offset of 0, and is 128
bytes long and a request for starting address 10 with a length of 5 words is received, the
20th through 29th byte within the table are sent to the client.
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.
Primary table Address Range Data type Type of Access
Discrete Inputs 1000-1FFF Single bit Read-only
Coils 2000-2FFF Single bit Read-write
Input Registers 3000-3FFF 16-bits Read-only
Holding Registers 4000-4FFF 16-bits Read-write
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