ProSoft Technology MVI56E-MCM/MCMXT - Manual (Page 118)

Reference MVI56E-MCM ♦ ControlLogix Platform
User Manual Modbus Communication Module
Page 118 of 199 ProSoft Technology, Inc.
June 18, 2014
6.2 Functional Overview
6.2.1 About the Modbus Protocol
Modbus is a widely-used protocol originally developed by Modicon in 1978. Since
that time, the protocol has been adopted as a standard throughout the
automation industry.
The original Modbus specification uses a serial connection to communicate
commands and data between Master and Slave devices on a network. Later
enhancements to the protocol allow communication over other types of networks.
Modbus is a Master/Slave protocol. The Master establishes a connection to the
remote Slave. When the connection is established, the Master sends the Modbus
commands to the Slave. The MVI56E-MCM module can work as a Master and as
a Slave.
The MVI56E-MCM module also works as an input/output module between itself
and the Rockwell Automation backplane and ControlLogix processor. The
module uses an internal database to pass data and commands between the
processor and Master and Slave devices on Modbus networks.
6.2.2 Backplane Data Transfer
The MVI56E-MCM module communicates directly over the ControlLogix
backplane. Data is paged between the module and the ControlLogix processor
across the backplane using the module's input and output images. The update
frequency of the images is determined by the scheduled scan rate defined by the
user for the module and the communication load on the module. Typical updates
are in the range of 2 to 10 milliseconds.
This bi-directional transference of data is accomplished by the module filling in
data in the module's input image to send to the processor. Data in the input
image is placed in the Controller Tags in the processor by the ladder logic. The
input image for the module is set to 250 words. This large data area permits fast
throughput of data between the module and the processor.
The processor inserts data to the module's output image to transfer to the
module. The module's program extracts the data and places it in the module's
internal database. The output image for the module is set to 248 words. This
large data area permits fast throughput of data from the processor to the module.