Data Flow Between The Module And Processor; Slave Mode - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module
5.4

Data Flow Between the Module and Processor

The following topics describe the flow of data between the two pieces of hardware
(CompactLogix or MicroLogix 1500-LRP processor and MVI69E-MBS module) and other
nodes on the Modbus network. You can configure each port on the module to emulate a
Modbus Master device or a Modbus Slave device.

5.4.1 Slave Mode

In Slave Driver mode, the MVI69E-MBS module responds to read and write commands
issued by a master on the Modbus network. The following diagram shows the data flow
for normal Slave mode.
Description
Step
Any time the module restarts (boots or reboots), the Modbus slave port driver receives
1
configuration information from the MBS controller tags. This information configures the
serial ports 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.
A Modbus Master device, such as a Modicon PLC or an HMI application, issues a read or
2
write command to the module's node address. The port driver qualifies the message before
accepting it into the module. Rejected commands cause an Exception Response.
After the module accepts the command, the data is immediately transferred to or from the
3
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.
After Steps 2 and 3 have been completed, either a normal response message or an
4
Exception Response message is sent to the Master.
Counters are available in the Status Block to permit the ladder logic program to determine
5
the level of activity of the Slave driver.
In Slave Pass-Through mode, write commands from the Master are handled differently than
they are in Normal mode. In Slave Pass-Through mode, all write requests are passed
directly to the processor and data is not written directly into the module's database.
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