User`s manual

B-8 Motion Mate™ DSM314 for Series 90™-30 PLCs User's Manual – January 2001 GFK-1742A
B
Table B-2. 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 PLC 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.
•
Note that the Series 90-30 COMM REQ (unlike many of the other Series 90-30 PLC
instructions) does not have an OK output.
•
It is necessary to initialize the data in the Command Block prior to executing the COMM
REQ instruction. Since the normal PLC 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 loaded via a Word-
type MOVE instruction, and the remaining decimal values are loaded via Integer-type
MOVEs.