User Manual
Appendix B
GFK-1742F
Jan 2020
DSM314 Communications Request Instructions
392
B-1.1
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 host controller 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:
Figure 179: 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.
The Command Block: The Command Block consists of several words of host controller
memory that contain additional information about the communications request. This
information includes timing parameters, a pointer to the Status Word, a Data Block,
memory types and sizes, and a specific command code. The Data Block specifies the
direction of the data transfer (via the Command Code) and location and type of data to be
transferred.
The Status Word: The Status Word is a single location in host controller data memory where
the CPU reports the result of the communications request. The Status Word address is
specified in the Command Block by the user. The following table lists the status codes
reported in the Status Word: