User`s manual

DSM314 COMM REQ
GFK-1742A Appendix B DSM314 Communications Request Instructions B-9
B
Section 3: The User Data Table (UDT) COMM REQ
The DSM314 has an 8192-byte memory area called the User Data Table (UDT) that is designated
for use with Local Logic (LL) programs. LL Programs can access all or part of this memory to
store and retrieve data. The UDT is useful for storing and retrieving large amounts of data such
as large batches of setup data.
The PLC CPU can write to or read from the UDT via a User Data Table Communications Request
(UDT COMM REQ) instruction in the PLC ladder program. A single UDT COMM REQ reads or
writes 2048 bytes of memory at a time. Therefore, the UDT is logically divided into four 2048
byte segments, called Segments 1-4, that can be accessed individually by a UDT COMM REQ.
There is a unique Read and a unique Write command for each of the four Segments, for a total of
8 possible UDT COMM REQ commands.
User Data Table COMM REQ Features and Usage Information
•
Reads or Writes 2 K (2048) bytes at a time to the Local Logic User Data Table. No other
value is permitted.
•
Only works with the DSM314 module (will not work with the DSM302)
•
Cannot be used to download parameter data to the DSM314
•
This instruction adds about 15 ms to PLC scan (sweep) time for one scan if the PLC
Communication Window Sweep Control parameter is set to COMPLETE (Run to
Completion). If the PLC Communication Window Sweep Control parameter is set to
LIMITED, the COMM REQ will be executed over several scans, with a smaller impact on
scan time. However, the COMM REQ probably will not be executed repeatedly – it will
only be executed when there is a need to change data. Therefore, if it was sent on the First
Scan, or during a job setup, it would not have an impact while the application is running.
•
To avoid memory access conflicts, it is recommended that a Periodic Subroutine not be used
during the time this COMM REQ is active.
•
This COMM REQ does not support discrete memory for its PLC Data Type.