ProSoft Technology MVI94-MCM-MHI - Manual (Page 72)

Reference MVI94-MCM ♦ Flex I/O Platform
User Manual Serial Communications Modbus Communication Module
Page 72 of 109 ProSoft Technology, Inc.
March 29, 2011
5.3 Functional Overview
The MVI94-MCM communication module interfaces Modbus master and slave
devices with the Flex I/O system. The module contains a virtual Modbus
database that is defined by the user. When configured as a master, this database
is used for the request and command messages sent from the Modbus master
port to Modbus slave devices. As a slave this database serves data to the master
device and passes control data to the processor via the backplane. Data areas in
the virtual Modbus database can be reserved for status and error information
generated by the module under user control.
The virtual Modbus database also interfaces with the Flex I/O system using the
Flex I/O bus (backplane). Data is made available to the processor on a
ControlNet network using this backplane interface. Input and output image tables
in the module present the data in the virtual Modbus database to the backplane.
A Modbus master port is present on the communication module to continuously
poll Modbus slave devices. Up to 100 user-defined commands can be defined for
the port. Data read from Modbus slave devices are placed in the virtual Modbus
database. Any write requests for the Modbus slave devices are sourced with data
from the virtual Modbus database.
The module can be configured to place slave devices that are not responding to
commands at a lower priority. If the module recognizes a slave device has failed
to respond to a message after the user defined retry count, it will mark the slave
as "in communication failure" and set the error delay counter to the specified
value. Each time the module encounters this slave in the command list, the
counter will be decremented. When the value reaches zero, the slave will be
placed in an active status. This facility can improve communication throughput on
the Modbus network.
Commands can be activated in the module under processor control. This feature
permits the processor to issue a command in the command list under program
control. When a command is activated, it will be placed in the command queue
for immediate execution. Normal command polling will begin after the command
queue is completely processed.
Additionally, the processor can send a command directly to any slave attached to
the Modbus master port. This feature can be used for commands that are not
issued on a regular basis and are not in the command list. Commands submitted
as events are placed in the command queue for immediate execution. They will
preempt normal operation of the poll list. When the command queue is
completely empty, normal command polling will resume.
Data in the virtual Modbus database is accessible through a configured Modbus
slave port. Remote Modbus master devices can monitor and control data in this
database through this port.
The module provides a Configuration/Debug port for use with an external
computer executing a terminal emulation program. The terminal emulation
program provided with the module permits uploading and downloading of the
configuration information required by the module. Additionally, the
Configuration/Debug port provides a view into the virtual Modbus database,
communication statistics and the configuration.