MVI56E-MCM ♦ ControlLogix Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 135 of 209
Pass-Through Blocks
The Pass-through Mode allows a Modbus Slave port to pass write commands
received from a host directly across the backplane to the ControlLogix processor
for handling by ladder logic. Although this feature requires more ladder logic in
order to implement a solution, there are certain situations where this functionality
can be useful. Some of these situations include:
1 When the slave needs to know when it has been written to
2 When the acceptance of data may require some conditioning
3 When the host’s write data registers must overlap the read register space
Unformatted Pass-Through Blocks (9996)
If one or more of the Slave Ports on the module are configured for the
unformatted pass-through mode, the module will pass blocks with identification
codes of 9996 to the processor for each received write command. Any Modbus
function 5, 6, 15, and 16 commands will be passed from the port to the processor
using this block identification number. Ladder logic must handle the receipt of all
Modbus write functions to the processor and to respond as expected to
commands issued by the remote Modbus Master device. The structure of the
unformatted Pass-through block is shown in the following table.
Pass-Through Block 9996 from Module to Processor
Number of bytes in Modbus message
The ladder logic will be 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 block with a write
block with the following format.
Response Block 9996 from Processor to Module
This will inform the module that the command has been processed and can be
cleared from the pass-through queue.
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