MVI56-MCM
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ControlLogix Platform
Reference
Modbus Communication Module
Page 88 of 159
ProSoft Technology, Inc.
July 24, 2008
8.2.2 About the MODBUS Protocol
MODBUS is a widely-used protocol originally developed by Modicon in 1978.
Since that time, the protocol has been adopted as a standard throughout the
automation industry.
The original MODBUS specification uses a serial connection to communicate
commands and data between master and slave devices on a network. Later
enhancements to the protocol allow communication over other types of networks.
MODBUS is a master/slave protocol. The master establishes a connection to the
remote slave. When the connection is established, the master sends the
MODBUS commands to the slave. The MVI56-MCM module works both as a
master and as a slave.
The MVI56-MCM module acts as an input/output module between devices on a
MODBUS network and the Rockwell Automation backplane. The module uses an
internal database to pass data and commands between the processor and the
master and slave devices on the MODBUS network.
8.2.3 Main Logic Loop
Upon completing the power up configuration process, the module enters an
infinite loop that performs the following functions:
8.2.4 ControlLogix
Processor Not in Run
Whenever the module detects that the processor has gone out of the Run mode
(for example, Fault or PGM), the Modbus ports can be shut down as prescribed
in the user configuration. When the processor is returned to a running state, the
module will resume communications on the network.
