ProSoft Technology MVI69E-MBTCP - Manual (Page 138)

Reference MVI69E-MBTCP ♦ CompactLogix Platform
User Manual Modbus TCP/IP Enhanced Communication Module
Page 138 of 150 ProSoft Technology, Inc.
May 20, 2015
7.4.12 Pass-Through Formatted Block for Functions 6 and 16 with
Word Data Block (9956)
If the server port on the module is configured for formatted Pass-Through mode,
the module sends input image blocks with identification codes of 9956, 9957,
9958 or 9959 to the processor for each write command received. Any incoming
Modbus Function 5, 6, 15 or 16 command is passed from the port to the
processor using a block identification number that identifies the Function Code
received in the incoming command.
The MBTCP Add-On Instruction handles the receipt of all Modbus write functions
and responds as expected to commands issued by the remote Modbus client
device.
Note: Mutual exclusion on Pass-Through Block IDs 9956, 9957, 9958, and 9959 from all server
connections. When multiple server connections are active and they receive write commands with
the same Function Code, the same block identifier from the above list is needed. The module
processes the command from the server which first received a command.
The module returns an Exception Code error code 6 (Node is busy - retry command later error)
from the other server that received the command last. The client retries the command on the busy
port after a short delay. This prevents Pass-Through blocks on multiple servers from overwriting
each other.
Block 9956: Request from Module to Processor
Offset Description
0 Read Block ID 9956
1 Write Block ID 9956
2 Number of word registers in the Modbus data set
3 Starting address for the Modbus data set
4 to 53 Modbus Data
54 to (n-1) Spare
Where n = 60, 120, or 240 depending on the Block Transfer Size parameter.
The ladder logic is responsible for parsing and copying the received message
and performing the proper control operation as expected by the client device.
The processor must then respond to the Pass-Through control block with an
output image write block with the following format. This informs the module that
the command has been processed and can be cleared from the Pass-Through
queue.
Block 9956: Response from Processor to Module
Offset Description
0 Write Block ID 9956
1 to (n-1) Spare