Read And Write Block Transfer Sequences - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module

5.3.3 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:
Read Register Count:
Write Register Start:
Write Register Count:
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).
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MVI69E-MBS Backplane Data Exchange
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