ProSoft Technology MVI69L-MBS - Manual (Page 72)

MVI69L-MBS Backplane Data Exchange MVI69L-MBS ♦ CompactLogix Platform
User Manual Communication Module
Page 72 of 154 ProSoft Technology, Inc.
January 6, 2014
4.4 Data Flow Between the Module and Processor
The following topics describe the flow of data between the two pieces of
hardware (CompactLogix processor and MVI69L-MBS module) and other nodes
on the Modbus network. Each port on the module can be configured to emulate a
Modbus Master device or a Modbus Slave device.
4.4.1 Slave Mode
The Slave Driver Mode allows the MVI69L-MBS module to respond to read and
write commands issued by a master on the Modbus network. The following
diagram shows the data flow for normal slave mode.
Step Description
1 Any time the module restarts (boots or reboots), the Modbus slave port driver receives
configuration information from the MBS controller tags. This information configures the
application port and defines slave node characteristics. The configuration information may
also contain instructions to offset data stored in the database to addresses different from
addresses requested in the received messages.
2 A Modbus Master device, such as a Modicon PLC or an HMI application, issues a read or
write command to the module’s node address. The port driver qualifies the message before
accepting it into the module. Rejected commands will cause an Exception Response.
3 After the module accepts the command, the data is immediately transferred to or from the
module’s internal database. On a read command, the data is read from of the database and
a response message is built. On a write command, the data is written directly into the
database and a response message is built.
4 After Steps 2 and 3 have been completed, either a normal response message or an
Exception Response message is sent to the Master.
5 Counters are available in the Status Block to permit the ladder logic program to determine
the level of activity of the Slave driver.