User`s manual

B-2 Motion Mate™ DSM314 for Series 90™-30 PLCs User's Manual – January 2001 GFK-1742A
B
Section 1: Communications Request Overview
The Communications Request uses the parameters of the
COMM REQ Ladder Instruction
and an
associated
Command Block
to define the characteristics of the request. An associated
Status Word
reports the results of each request.
Structure of the Communications Request
The Communications Request is made up of three main parts:
•
The COMM REQ Ladder Instruction
•
The Command Block, which is a block of PLC memory (usually %R memory) that contains
instructions and data for the COMM REQ.
•
The Status Word, which is one word of memory that status/error codes are written to.
The figure below illustrates the relationship of these parts:
a44916
INPUTS
AND
OUTPUTS
FOR COMREQ
INSTRUCTION
COMMAND
BLOCK
POINTER
COMREQ
INSTRUCTION
DETAILS
OF THE
REQUEST
STATUS
WORD
POINTER
COMMAND
BLOCK
STATUS
WORD
ERROR
CODES
Figure B-1. Structure of the COMM REQ
The COMM REQ Ladder Instruction:
The COMM REQ Ladder Instruction is the main structure
used to enter specific information about a communications request. This information includes the
rack and slot location of the DSM module associated with the request, and a parameter that points
to the starting address of the Command Block. Note that in programming this instruction, the
command block data should be initialized in the ladder program before the rung containing the
COMM REQ instruction is executed.