MVI56-DH485R
♦
ControlLogix Platform
Reference
DH485 Interface Module for Remote Chassis
User Manual
Page 58 of 97
ProSoft Technology, Inc.
August 27, 2009
5.2.2 Backplane Data Transfer
The MVI56-DH485R 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 for the input and
output images. Typical updates are in the range of 5 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 42 words. This data area permits fast
throughput of data between the module and the processor.
The processor inserts data in 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 42 words. This data
area permits fast throughput of data from the processor to the module.
The following illustration shows the data transfer method used to move data
between the ControlLogix processor, the MVI56-DH485R module, and the DH-
485 network.
DH485
Port
Driver
Master
Driver
Logic
Master
Driver
Logic
Slave
Driver
Logic
Slave
Driver
Logic
Module’s
Internal
Database
MVI56-DH485R Module
To DH485
Network
Ladder
Logic
Transfers
Data from
module’s input
image to data
areas in the
processor
ControlLogix Processor
Controller Tags
ControlLogix Processor
Ladder
Logic
Transfers
Data from
Processor
data areas
to output image
Input Image
Output image
Status
Read Data
Write Data
Special Control
Blocks
All data transferred between the module and the processor over the backplane is
through the input and output images. Ladder logic must be written in the
ControlLogix processor to interface the input and output image data with data
defined in the controller tags. All data used by the module is stored in its internal
database.
