ProSoft Technology MVI56E-MCMR - Manual (Page 148)

Reference MVI56E-MCMR ♦ ControlLogix Platform
User Manual Modbus Communication Module with Reduced Data Block
Page 148 of 225 ProSoft Technology, Inc.
May 13, 2014
6.3 Functional Overview
6.3.1 Processor/Module Data Transfers
The MVI56E-MCMR module communicates directly over the ControlLogix
backplane or across any supported 1756 network bridge (the most common
being 1756-CNBx Control Net Bridge and 1756-ENxT EtherNet/IP Bridge). Data
travels between the module and the ControlLogix processor across the
backplane or network using the module's Input and Output Images and MSG
instructions.
The data update frequency is determined by the Requested Packet Interval (RPI)
defined in the module's I/O configuration and the communication load and speed
on the Modbus, ControlNet, or EtherNet/IP networks. When the module is
installed in the ControlLogix chassis, typical backplane update rates range from 1
to 10 milliseconds. Execution time for MSG instruction data transfers are
dependant on the amount of unscheduled time available on the backplane or
network and how frequently the MSG instruction is executed.
Data received by the Modbus driver is placed in the module's internal memory in
an area designated to receive it. The data in this area is then transferred to the
processor in the I/O Input Image. This data is processed by ladder logic to fill the
MCMR.DATA.R
EAD D ATA array controller tags. The Input Image size is 42 words
per image block, 40 of which are user data, along with two control words. Larger
amounts of user data can be moved from the module to the processor by using
multiple sequential block transfers. The module will automatically sequence
multiple 40-word blocks until the total amount of user data has been moved. The
module calculates the required number of read data blocks by dividing the R
EAD
R EGISTER C OUNT parameter in the configuration file by 40 and rounding up to the
next higher whole integer value.
The processor inserts data in the module's Output Image to transfer to the
module. The module's program extracts the data and stores it in the internal
module database, so that it may be transmitted by the Master driver to Slaves on
serial network. Additionally, the ControlLogix processor can send special function
blocks to the module to instruct it to perform special tasks. The Output Image
size is 42 words per image block, 40 of which are user data, along with one
control word. Larger amounts of user data can be moved from the processor to
the module by using multiple sequential block transfers. The module will
automatically sequence multiple 40-word blocks until the total amount of user
data has been moved. The module calculates the required number of write data
blocks by dividing the Write Register Count parameter in the configuration file by
40 and rounding up to the next higher whole integer value.
Special function blocks are also passed between the module and the processor
using MSG instructions initiated under ladder logic control. These blocks are
transferred between the processor and the module only when triggered by user-
programmed logic.