User Manual
Appendix B
GFK-1742F
Jan 2020
DSM314 Communications Request Instructions
397
TASK: The number 0 should always be entered here for a DSM module.
FT Output:
The function’s FT (fault) output can provide an output to optional logic that can
verify successful completion of the Communications Request. The FT output can have these
states:
Table 83: COMM REQ Instruction FT Output Truth Table
FT Output
Enable Input Status Does an Error Exist? FT output
Active
No
Low
Active
Yes
High
Not active
No execution
Low
•
The FT output will be set High if:
—
The specified target address is not present (for example, specifying Rack 1
when the system only uses Rack 0).
—
The specified task number is not valid for the device (the TASK number should
always be 0 for the DSM).
—
Data length is set to 0.
DSM COMM REQ Programming Requirements and Recommendations
•
It is recommended that DSM COMM REQ instructions be enabled with a contact
from a transition coil.
•
If using more than one DSM COMM REQ in a ladder program, verify that a previous
COMM REQ executed successfully before executing another one. This can be done
by checking the Status Word and the FT (Fault) output, explained earlier in this
appendix under the heading “Monitoring the Status Word.”
•
As seen in the table above, the FT output will be held False if the Enable Input is not
active. This means that if the COMM REQ is enabled by a transitional (one-shot)
contact and a fault occurs, the FT output will only be High for one host controller
scan. Therefore, to “capture” the fault, you can program the fault output as a Set
coil, which would not be automatically reset at the end of a scan. Additional logic
would then be needed to reset the fault output coil after the fault is acknowledged.
•
Programming a device, such as a Set Coil, on the FT output of the COMM REQ is
optional.
•
It is necessary to initialize the data in the Command Block prior to executing the
COMM REQ instruction. Since the normal host controller sweep order is from top to
bottom, initializing the Command Block in an earlier rung (or rungs) than the rung
that contains the COMM REQ will facilitate this requirement. See the example at the
end of this appendix.
•
Recommendation: If you use MOVE instructions to load values into Command Block
registers, use a Word-type MOVE to load a hexadecimal number, and an Integer-
type MOVE to load a decimal number. You will see this applied in the example at the
end of this appendix for a Parameter Load COMM REQ, where the E501h code is