ProSoft Technology MVI71-MCM - Manual (Page 74)

MVI71-MCM ♦ PLC Platform Reference
Modbus Communication Module
Page 74 of 111 ProSoft Technology, Inc.
December 28, 2007
5.2.10 Pass-Through Control Blocks
The pass-through feature allows a Master device to write commands directly to
the PLC ladder logic.
Formatted Pass-Through Control Blocks
If one or more of the slave ports on the module are configured for the formatted
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
or 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 formatted pass-through
control block is shown in the following tables:
Function 5
Offset Description Length
0 0 1
1 9958 1
2 1 1
3 Bit Address 1
4 Data 1
5 to 58 Modbus message received 59
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 control block with
a write block with the following format.
Offset Description Length
0 9958 1
1 to 63 Spare 63
This will inform the module that the command has been processed and can be
cleared from the pass-through queue.
Function 6 and 16
Offset Description Length
0 0 1
1 9956/9957 (Floating-point) 1
2 Number of data words 1
3 Data Address 1
4 to 63 Data 60
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 control block with
a write block with the following format.