Reference MVI56-MCM
♦
ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc.
Page 103 of 159
July 24, 2008
Master Driver Mode
In the Master mode, the MVI56-MCM module issues read or write commands to
slave devices on the Modbus network. These commands are user configured in
the module via the Master Command List received from the ControlLogix
processor or issued directly from the ControlLogix processor (event command
control). Command status is returned to the processor for each individual
command in the command list status block. The location of this status block in
the module's internal database is user defined. The following flow chart and
associated table describe the flow of data into and out of the module.
Step Description
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
command 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 network 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 Integrating the Sample Ladder Logic into an Existing Project (page 142)
for a complete description of the parameters required to define the virtual
Modbus master port. Refer to the
MCM Driver
documentation for a complete
discussion of the structure and content of each command. Care must be taken in
constructing each command in the list for 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.
