ProSoft Technology MVI69-MCM - Manual (Page 115)

MVI69-MCM ♦ CompactLogix or MicroLogix Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 115 of 167
March 22, 2011
5.4 Normal Data Transfer
Normal data transfer includes the paging of the user data found in the module’s
internal database in registers 0 to 4999 and the status data. These data are
transferred through read (input image) and write (output image) blocks. The
following topics describe the structure and function of each block:
5.4.1 Block Request from the Processor to the Module
These blocks of data transfer information from the processor to the module. The
structure of the output image used to transfer this data is shown below:
Offset Description Length (words)
0 Write Block ID 1
1 to (n) Write Data (n)
(n) = 60, 120, or 240 depending on the Block Transfer Size parameter (refer to the configuration
file).
The Write Block ID is an index value used to determine the location in the
module’s database where the data will be placed.
5.4.2 Block Response from the Module to the Processor
These blocks of data transfer information from the module to the processor. The
structure of the input image used to transfer this data is shown below:
Offset Description Length (words)
0 Read Block ID 1
1 Write Block ID 1
2 to (n+1) Read Data (n)
(n) = 60, 120, or 240 depending on the Block Transfer Size parameter (refer to the configuration
file).