ProSoft Technology MVI94-MCM-MHI - Manual (Page 82)

Reference MVI94-MCM ♦ Flex I/O Platform
User Manual Serial Communications Modbus Communication Module
Page 82 of 109 ProSoft Technology, Inc.
March 29, 2011
Master Command List
The MVI94-MCM communication module’s primary services are data
concentration and communication gateway. The Modbus master port polls
Modbus slave devices based on user defined commands and places the data in
the virtual Modbus database. The Flex I/O bus interfaces with this database to a
Flex processor. The user is responsible for defining the structure and content of
the virtual Modbus database.
In order to interface the virtual Modbus database with Modbus slave devices, you
must construct a command list. The commands in the list specify the Modbus
slave device to be utilized, the function to be performed (read or write), the data
area in the device to interface with and the position in the virtual Modbus
database to be associated with the device data. Up to 100 commands can be
entered for this purpose. The list is processed from top (command #0) to bottom.
A poll interval parameter is associated with each command to specify a minimum
delay time in seconds between the issuance of a command. If the user specifies
a value of 10 for the parameter, the command will be executed no more
frequently than every 10 seconds. Additionally, a user specified time delay could
be inserted between the issuance of each command. This is useful for slow
responding slave devices.
Write commands have a special feature, as they can be set to execute only if the
data in the write command changes. If the data in the command has not changed
since the command was last issued, the command will not be executed. If the
data in the command has changed since the command was last issued, the
command will be executed. Use of this feature can lighten the load on the
Modbus network. In order to implement this feature; set the enable code for the
command to a value of 2.