ProSoft Technology MVI56E-MCMR - Manual (Page 151)

MVI56E-MCMR ♦ ControlLogix Platform Reference
Modbus Communication Module with Reduced Data Block User Manual
ProSoft Technology, Inc. Page 151 of 225
May 13, 2014
6.3.2 Normal Data Transfer Blocks
Normal data transfer includes the transferring of data received by, or to be
transmitted to, the Modbus drivers and the status data. This data is transferred
through read (Input Image) and write (Output Image) blocks. Refer to Module
Data (page 118) for a description of the data objects used with the blocks and the
ladder logic required. The following topics discuss the structure and function of
each block.
Read Block
Read Blocks transfer data from the module to the ControlLogix processor's
MCMR.DATA.R
EAD D ATA controller tag array. The following table describes the
structure of the input image.
Read Block from Module to Processor
Word Offset Description Length
0 Write Block ID (1 to 125) 1
1 to 40 Read Data 40
41 Read Block ID (1 to 125) 1
The Read Block Identification Code (word 41) is used to signal to the
ControlLogix processor that a new block is ready for processing. It also tells the
processor where in the MCMR.DATA.R
EAD D ATA controller tag array to place the
data contained in the block.
If the value of the code is set to 1, the block contains the 40 words of data from
the module database starting at the address specified in the configuration file
parameter, R
EAD S TART R EGISTER . This data will be put into the ReadData array,
starting at R EAD D ATA [0] up to R EAD D ATA [39].
Read Block ID 2 would contain the next consecutive 40 words from the module
database to be placed in R
EAD D ATA [40] up to R EAD D ATA [79] and so on, up to
the total amount of data words specified in the configuration parameter, R EAD
R EGISTER C OUNT .
The block also contains the Write Block Identification Code the module expects
to receive from the processor. Under normal data transfer conditions, the ladder
logic should use the Write Block Identification Code to build the appropriate
Output Image data block, unless a special function block is required. The special
function blocks will be discussed in the next section.