MVI69E-MBS ♦ CompactLogix™ Platform
Reference
Modbus Serial Enhanced Communication Module
User Manual
ProSoft Technology, Inc.
Page 145 of 159
8.4.13 Pass-through Formatted Word Data Block for Functions 6 & 16 (9956)
Block 9956: Pass-Through Formatted Block for Functions 6 and 16 with Word Data
Block
If one or more of the slave ports on the module are 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 MBS Add-On Instruction handles the receipt of all Modbus write functions and to
respond to commands issued by the remote Modbus Master device.
Note: Mutual exclusion on Pass-Through Block IDs 9956, 9957, 9958, and 9959 from both ports - If both
ports are configured as slave ports, when both of the slave ports receive write commands with the same
Function Code, which would need to use the same block identifier from the above list, the module will
process the command from the port which first received the command and will return an Exception Code
error code 6 (node is busy - retry command later error) from the other port that received the command last.
The Master retries the command on the busy port after a short delay. This prevents Pass-Through blocks on
both ports 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 Modbus data set
3
Starting address for Modbus data set
4 to
(n+1)
Data (Length in words = n - 2)
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 Master 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
Spare (Length in words = n - 2)