Read and write block transfer sequences – ProSoft Technology MVI69E-MBS User Manual
Page 73
MVI69E-MBS ♦ CompactLogix Platform
MVI69E-MBS Backplane Data Exchange
Communication Module
User Manual
ProSoft Technology, Inc.
Page 73 of 162
January 6, 2014
4.3.3 Read and Write Block Transfer Sequences
The Read Block ID is an index value that determines the location where the data
is placed in the processor controller tag array of module read data. The number
of data words per transfer depends on the configured Block Transfer Size
parameter in the configuration file (possible values are 60, 120, or 240).
The Write Block ID associated with the block requests data from the processor.
Under normal program operation, the module sequentially sends read blocks and
requests write blocks. For example, if the application uses three read and two
write blocks, the sequence is as follows:
R1W1
R2W2
R3W1
R1W2
R2W1
R3W2
R1W1
This sequence continues until interrupted by other write block numbers sent by
the controller or by a command request from a node on the Modbus network or
operator control through the module’s Configuration/Debug port.
The following example shows a typical backplane communication application.
If the backplane parameters are configured as follows:
Read Register Start:
0
Read Register Count:
480
Write Register Start:
480
Write Register Count:
480
The backplane communication would be configured as follows:
Database address 0 to 479 is continuously transferred from the module to the
processor. Database address 480 to 959 is continuously transferred from the
processor to the module.
The Block Transfer Size parameter configures how the Read Data and Write
Data areas are broken down into data blocks (60, 120, or 240).