ProSoft Technology MVI56E-MCM/MCMXT - Manual (Page 137)

MVI56E-MCM ♦ ControlLogix Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 137 of 199
June 18, 2014
The following illustration describes the flow of data to and from the module.
1 The Master driver obtains configuration data from the ControlLogix processor.
The configuration data obtained includes the number of commands and the
Master Command List. These values are used by the Master driver to
determine the type of commands to be issued to the other nodes on the
Modbus network.
2 After configuration, the Master driver begins transmitting read and/or write
commands to the other nodes on the network. If writing data to another node,
the data for the write co
mmand is obtained from the module’s internal
database to build the command.
3 Presuming successful processing by the node specified in the command, a
response message is received into the Master driver for processing.
4 Data received from the node on the netwo rk is passed into the module’s
internal database, assuming a read command.
5 Status is returned to the ControlLogix processor for each command in the
Master Command List.
Refer to Using the Sample Program in an Existing Application (page 186) for a
complete description of the parameters required to define the virtual Modbus
Master Port.
Take care when constructing each command to ensure predictable operation of
the module. If two commands write to the same internal database address of the
module, the results will not be as desired. All commands containing invalid data
are ignored by the module.