User`s manual

B-6 Motion Mate™ DSM314 for Series 90™-30 PLCs User's Manual – January 2001 GFK-1742A
B
Operation of the Communications Request
The figure below illustrates the flow of information from the PLC CPU to the DSM module:
FIRMWARE
INSTRUCTIONS
ON-BOARD
MEMORY
DSM
MODULE
LADDER
PROGRAM
PLC CPU
COMREQ
CPU
MEMORY
DATA
STATUS
WORD
a44917a.cvs
BACKPLANE
DATA
COMMAND
STATUS
Figure B-2. Operation of the DSM Communications Request
A Communications Request is initiated when a COMM REQ ladder instruction is activated
during the PLC scan. At this time, details of the Communications Request, consisting of
command and data, are sent from the PLC CPU to the DSM module.
•
In the case of a Parameter Load COMM REQ, the command data specifies that data is to be
read from PLC memory and copied into specific DSM parameter memory locations.
•
In the case of a UDT COMM REQ, the command data either specifies that data is to be read
from PLC memory and copied into a specific UDT memory Segment, or read from a
specific UDT memory Segment and copied into PLC memory.
The order in which these instructions are sent is critical, so the Command Block for each type of
COMM REQ should be programmed exactly as instructed later in this appendix. In the figure
above, the DSM module is shown in the CPU rack and communications occur over the PLC
backplane. If the DSM module is located in an expansion or remote rack, the commands and data
are sent over the CPU rack’s backplane, through the expansion or remote cable to the rack
containing the DSM module, and across that rack’s backplane to the DSM.
At the conclusion of every request, the PLC CPU reports the status of the request to the Status
Word, which is a location in PLC memory that is designated by the Status Word Pointer in the
Command Block.