A.4 Data model

The Modbus standard defines bit-addressable and 16-bit word addressable input and output data items. Modbus bases its data model on a series of tables that have distinguishing characteristics. The four primary tables for data model are as following.

Table A.4-1: Modbus data model table

Primary tables	Object type	Type of access	Comments
Discretes Input	Single bit	Read-Only	This type of data can be provided by an I/O system, e.g. read the status of switch
Coils	Single bit	Read-Write	This type of data can be alterable by an application program, e.g. switch on a transducer
Input Registers	16-bit word	Read-Only	This type of data can be provided by an I/O system, e.g. read temperature on a sensor
Holding Registers	16-bit word	Read-Write	This type of data can be alterable by an application, e.g. set value to a controller

There are two ways of organizing the data in device. Each device can have its own organization of the data according to its application. Figure A.4-1 below shows an example for data organization in a device having digital and analog, inputs and outputs. Data block (device application memory) is accessible with different Modbus functions, such as read coils, write holding registers. All the data elements handled via Modbus can be located in device application memory by reference numbers form 1to n. The pre-mapping between the Modbus data model and the device application is totally vendor device specific.

Figure A.4-1: Implementation example of Modbus data model