Users manual

22-10
SCADA 3000 User’s Manual
Month(Now) ' Holds the present value from 1 to 12.
Day(Now) ' Holds the present value from 1 to 31.
Year(Now) ' Holds the present value in 4 digit format (e.g. 2001).
Hour(Now) ' Holds the present value in 24 hour time from 0 to 23.
Minute(Now) ' Holds the present value from 0 to 59.
Second(Now) ' Holds the present value from 0 to 59.
Weekday(Now) ' Holds the present value from 0(Sunday) to 6(Saturday).
Example: This program will set DataPointValue4 to 35 at midnight.
if (Hour(Now)=0) and (Minute(Now)=0) and (Second(Now)=0) then
DataPointValue4.Value = 35
end if
Programming Methodology
There are three methods that will typically be used when creating a control system based on
a distributed control program. The method you choose will depend on your application, how
many units you have, and other factors which may be specific to your application. The three
methods are described below:
Method 1
Your distributed control program performs all control logic. No logic programs run on the
SCADA units themselves.
Advantages
• Thisallowsforcentralizedcontrolandthereisonlyonecontrolprogramtocre-
ate and maintain.
Disadvantages
• YouwillneedallI/Opointsrequiredforrunningyourlogicprogramtobeonthe
realtime screen.
• IfthePCcrashesortheprogrammustbehalted,thenallcontrolstops.
Method 2
Your distributed control program only passes I/O information from one unit to another. All
control logic is performed in the SCADA units themselves using a C or Ladder program.
Information can easily be read from unit A and written into the C or ladder variables of unit B.
Advantages
• ControlisperformedlocallyattheSCADAunit.IfthePCrunningthedistrib-
uted control program crashes or if the program must be halted, local control will
continue to function.
• ThisminimizesthenumberofI/Opointsonyourrealtimescreen.
Disadvantages
• YouwillneedtohaveseparatecontrolprogramsrunningforeachSCADAunit
which may be more complicated to write and maintain.
Method 3
AcombinationofMethods1&2whereboththedistributedcontrolprogramrunscontrol
logic and each SCADA unit also runs its own specific control program.