ProSoft Technology MVI71-MCM - Manual (Page 78)

MVI71-MCM ♦ PLC Platform Reference
Modbus Communication Module
Page 78 of 111 ProSoft Technology, Inc.
December 28, 2007
Master Driver Mode
In Master Mode, the MVI71-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 PLC processor or
issued directly from the PLC 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 PLC 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 (Refer to the Module Set Up section).
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 PLC processor for each command in the Master Command
List.