Reference MVI56-MCM ♦ ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 95 of 159
July 24, 2008
Offset Description Length
0 3001 or 3101 1
1 Number of Slaves in Block 1
2 to 201 Slave indexes 200
202 to 247 Spare 46
The module will respond with a block with the same identification code received
and indicate the number of slaves acted on with the block. The format of this
response block is displayed in the following table:
Offset Description Length
0 Reserved 1
1 Write Block ID 1
2 Number of slaves processed 1
3 to 248 Spare 246
249 3001 or 3101 1
8.2.10 Command Control Blocks
Command control blocks are special blocks used to control the module or
request special data from the module. The current version of the software
supports five command control blocks: event command control, command
control, write configuration, warm boot and cold boot.
Event Command
Event command control blocks send Modbus commands directly from the ladder
logic to one of the master ports. The format for these blocks is displayed in the
following table:
Offset Description Length
0 1000 to 1255 or 2000 to 2255 1
1 Internal DB Address 1
2 Point Count 1
3 Swap Code 1
4 Modbus Function Code 1
5 Device Database Address 1
6 to 247 Spare 242
The block number defines the Modbus port to be considered and the slave node
to be accessed. Blocks in the 1000 range are directed to Modbus Port 1, and
blocks in the 2000 range are directed to Modbus Port 2. The slave address is
represented in the block number in the range of 0 to 255. The sum of these two
values determines the block number. The other parameters passed with the
block construct the command. The Internal DB Address parameter specifies
the module's database location to associate with the command. The Point Count
parameter defines the number of points or registers for the command. The Swap
Code is used with Modbus function 3 requests to change the word or byte order.
The Modbus Function Code has one of the following values 1, 2, 3, 4, 5, 6, 15
or 16. The Device Database Address is the Modbus register or point in the
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