ProSoft Technology MVI56E-MNETC - Manual (Page 139)

MVI56E-MNETC ♦ ControlLogix Platform Reference
Modbus TCP/IP Client Enhanced Communication Module User Manual
ProSoft Technology, Inc. Page 139 of 183
February 3, 2013
2 A host device, such as a Modicon PLC or an HMI application, issues a read
or write command to the module’s node address. The server driver validates
the message before accepting it into the module. If the message is
considered invalid, an error response is returned to the originating Client
node.
3 After the module accepts the command, the module processes the data
contained in the command.
If the command is a read command, the data is read out of the database and
a response message is built.
If the command is a write command, the data is written directly into the
database and a response message is built.
If the command is a write command and the pass-through feature is utilized,
the write message is transferred to the processor ladder logic and is not
written directly into the module’s database, unless it is returned as a change
in the output image that overwrites data in the WriteData area as a result of
such ladder logic processing.
4 After the data processing has been completed in Step 3, a response is issued
to the originating Client node.
Counters are available in the Status Block that permit the ladder logic program to
determine the level of activity of the server driver.