turck FGEN-AIM Series, User Manual

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Implementation of Modbus TCP
D301271 1013 - FGEN - multi protocol 9-4
If an error related to the Modbus function requested occurs, the field contains an exception code that
the server application can use to determine the next action to be taken.
9.1.2 Data model
The data model distinguishes 4 basic data types:
For each of these basic data types, the protocol allows individual selection of 65536 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.
It’s obvious that all the data handled via Modbus (bits, registers) must be located in device application
memory.
Access to these data is done via defined access-addresses (see „Modbus registers“, page 9-7 ).
The example below shows the data structure in a device with digital and analog in- and outputs.
Figure 9-4:
Modbus data
transmission
(acc. to
Modbus-IDA)
Table 9-1:
Data types for
Modbus Data Type Object type Access Comment
Discrete Inputs Bit Read This type of data can be provided by an I/O system.
Coils Bit Read-Write This type of data can be alterable by an application
program.
Input
Registers 16-bit,
(word) Read This type of data can be provided by an I/O system.
Holding
Registers 16-bit,
(word) Read-Write This type of data can be alterable by an application
program.