turck FGEN-AIM Series, User Manual

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D301271 1013 - FGEN - multi protocol
9-3
Common Modbus description
9.1.1 Protocol description
The Modbus protocol defines a simple protocol data unit (PDU) independent of the underlying
communication layers.
The mapping of Modbus protocol on specific buses or network can introduce some additional fields on
the application data unit (ADU).
The Modbus application data unit is built by the client that initiates a Modbus transaction.
The function code indicates to the server what kind of action to perform.
The Modbus application protocol establishes the format of a request initiated by a client.
The field function code of a Modbus data unit is coded in one byte. Valid codes are in the range of 1...
255 decimal (128 – 255 reserved for exception responses).
When a message is sent from a Client to a Server device the function code field tells the server what kind
of action to perform. Function code "0" is not valid.
Sub-function codes are added to some function codes to define multiple actions.
The data field of messages sent from a client to server devices contains additional information that the
server uses to take the action defined by the function code. This can include items like discrete and
register addresses, the quantity of items to be handled, and the count of actual data bytes in the data
field.
The data field may be non-existent (= 0) in certain kinds of requests, in this case the server does not
require any additional information. The function code alone specifies the action.
If no error occurs related to the Modbus function requested in a properly received Modbus ADU the
data field of a response from a server to a client contains the data requested.
Figure 9-2:
Modbus tele-
gram acc. to
Modbus-IDA
Figure 9-3:
Modbus data
transmission
(acc. to
Modbus-IDA)