ProSoft Technology MVI94-DNP - Manual (Page 81)

MVI94-DNP ♦ Flex I/O Reference
DNP 3.0 Server over Ethernet Communication Module User Manual
ProSoft Technology, Inc. Page 81 of 131
December 17, 2010
The module and the processor constantly monitor input and output images. How
does either one know when a new block of data is available? Recognizing a
change in the header information of the image (word 0) solves the problem. For
example, when the module recognizes a different value in the first word of the
output image, new processor write data is available. When the processor
recognizes a new value in the first word of the input image, new processor read
data is available. This technique requires the storage of the previously processed
data block identification code.
The following illustration shows the normal sequence of events for data transfer:
1 The steps outlined in the diagram above are discussed below:
2 During program initialization, the write block identification code is set to 0
(binary input data) and read block identification codes are set to 20 (binary
output data). The last block write variable is set to -1.
3 The program copies the first six-word block of the binary output database to
the input image (words 2 to 7). It then sets the current write block code in
word 1 of the input image. To "trigger" the write operation, the program
places the current read block code into word 0 of the input image.
4 The Flex processor recognizes a new value in word 0 of the input image
(based on the last_read_block_code not equal to read_block_code) in its
ladder logic. The ladder logic computes the offset into the file based on the
data block number (defines the data type and offset of block for data type).
5 The new data contained in the input image (words 2 to 7) is copied to the
offset in the processor’s user data file. The last_read_block_code storage
register in the processor is updated with the new read_block_code.
Note: If the data area transferred from the module exceeds the size of a single user file in the Flex
processor, logic will be required to handle multiple files.