ProSoft Technology MVI69-MCM - Manual (Page 110)

Reference MVI69-MCM ♦ CompactLogix or MicroLogix Platform
User Manual Modbus Communication Module
Page 110 of 167 ProSoft Technology, Inc.
March 22, 2011
5.3 Data Flow between MVI69-MCM Module and CompactLogix or
MicroLogix Processor
The following topics describe the flow of data between the two pieces of
hardware (MVI69-MCM processor and MVI69-MCM module) and other nodes on
the MODBUS network under the module’s different operating modes. Each port
on the module is configured to emulate a MODBUS Master device or a MODBUS
slave device. The operation of each port depends on this configuration. The
sections below discuss the operation of each mode.
5.3.1 Slave Driver
The Slave Driver Mode allows the MVI69-MCM module to respond to data 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 Anytime the module restarts (boots or reboots), the MODBUS slave port driver receives
configuration information from a user defined .CFG file stored on the MVI69-MCM. 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.
2 A Host 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. If the command is a read command, the data is read from of
the database and a response message is built. If the command is a write command, the
data is written directly into the database and a response message is built.
4 After Steps 2 and 3 has been completed, either a normal response message or an
Exception Response message will be sent to the Master.
5 Counters are available in the Status Block that permit the ladder logic program to determine
the level of activity of the Slave Driver.