ProSoft Technology MVI71-MNET - Manual (Page 73)

Reference MVI71-MNET ♦ PLC Platform
User Manual Modbus TCP/IP Interface Module
ProSoft Technology, Inc.
Page 73 of 109
June 23, 2009
Pass-Through Control Blocks
If the module is set for pass-through operation by placing a value of 1 to3 in the
configuration file parameter Pass-Through Mode, the module will send special
blocks to the module when a write request is received from a client. Any Modbus
function 5, 6, 15 or 16 commands will be passed from the server to the processor
using this block identification numbers 9956 to 9961, 9970 and 9996. Ladder
logic must handle the receipt of these blocks and to place the enclosed data into
the proper controller tags in the module.
There are two basic modes of operation when the pass-through feature is
utilized: Unformatted (code 1) and Formatted (code 2 or 3). The unformatted
mode will pass the message received on the server directly to the processor
without any processing. The format of the read block transferred in this mode is
shown below:
Offset Description Length
50 0 1
51 9996 1
52 Number of bytes in Modbus msg 1
53 Reserved (always 0) 1
54 to 113 Modbus message received 60
The ladder logic should copy and parse the received message and control the
processor as expected by the master device. The processor must respond to the
pass-through control block with a write block with the following format.
Offset Description Length
50 9996 1
This will inform the module that the command has been processed and can be
cleared from the pass-through queue.
In formatted pass-through mode, the module will process the received write
request and generate a special block dependent on the function received. There
are two modes of operation when the formatted pass-through mode is selected. If
code 2 is utilized (no swap), the data received in the message is presented in the
order expected by the processor. If code 3 is utilized (swap mode), the bytes in
the data area of the message will be swapped. This selection is applied to all
received write requests. The block identification code used with the request is
dependent on the Modbus function requested. Block 9956 passes word type data
for functions 6 and 16. Block 9957 passes a floating-point message for functions
6 and 16. Block 9958 is utilized when Modbus function 5 data is received. Block
9959 is employed when function 15 is recognized. Block 9960 is used for
function 22 and Block 9961 is used for function 23 requests. Block 9970 is
utilized for function 99. The format for the read blocks is shown below:
Offset Description Length
50 0 1
51 9956, 9957, 9958, 9960 or 9961 1
52 Number of word registers in Modbus data set 1
53 Starting address for Modbus data set 1
54 to 113 Modbus data set 60