ProSoft Technology MVI71-MNET - Manual (Page 63)

Reference MVI71-MNET ♦ PLC Platform
User Manual Modbus TCP/IP Interface Module
ProSoft Technology, Inc.
Page 63 of 109
June 23, 2009
The Write Block ID associated with the block requests data from the PLC
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 will be as follows:
R1W1
→R2W2→R3W1→R1W2→R2W1→R3W2→R1W1→
This sequence will continue 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.
Write Block
These blocks of data transfer information from the PLC processor to the module.
The following table describes the structure of the output image.
Offset Description Length
0 Write Block ID 1
1 to 60 Write Data 60
61 to 63 Spare 3
The Write Block ID is an index value used to determine the location in the
module’s database where the data will be placed. Each transfer can move up to
60 words (block offsets 1 to 60) of data.
Initialize Output Data
When the module performs a restart operation, it will request blocks of output
data from the processor to initialize the module’s output data. Use the Initialize
Output Data parameter in the configuration file to bring the module to a known
state after a restart operation. The following table describes the structure of the
request block.
Offset Description Length
0 1000 to 1083 1
1 1000 to 1083 1
2 to 63 Spare 62
The block number in word 0 of the block determines the data set of up to 60
output words to transfer from the processor. Ladder logic in the processor must
recognize these blocks and place the correct information in the output image to
be returned to the module. The format of the returned write block is shown in the
following table.
Offset Description Length
0 1000 to 1083 1
1 to 60 Output Data 60
61 to 63 Spare 3