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MVI56E-MNETR ♦ ControlLogix Platform
Ladder Logic
Modbus TCP/IP Interface Module with Reduced Data Block
User Manual
ProSoft Technology, Inc.
Page 89 of 175
June 14, 2011
3.2
Modbus Message Data
During pass-through operation, write messages received at the MVI56E-MNETR
server write messages through to the processor. It is the responsibility of the
ladder logic to process the message received using this feature. Two data
objects are required for this mode: a variable to hold the length of the message
and a buffer to hold the message.
This information is passed from the module to the processor using a block
identification code of 9996 if the unformatted pass-through mode (code 1) is
selected as the pass through mode in the configuration file. Word one of this
block contains the length of the message and the message starts at word 3.
Other controller tags are required to store the controlled values contained in
these messages. The Modbus protocol supports control of binary output (coils -
functions 5 and 15) and registers (functions 6 and 16).
Additionally, formatted message blocks can be sent from the module to the
processor when the pass-through option is selected using the format selection
(codes 2 or 3 in the MNET.CFG file). These blocks require less decoding than
the unformatted blocks. Refer to Pass-Through Control Blocks (page 122) for a
full discussion on utilizing the pass-through option in an application.