Rockwell Automation Allen-Bradley DriveLogix 5730, User Manual

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3-4 Placing and Configuring Local I/O
Estimating RPI As you install modules, the minimum backplane RPI increases. The RPI
(request packet interval) defines the frequency at which the controller sends
and receives all I/O data on the backplane. There is one RPI for the entire
1769 backplane. Consider these guidelines when installing modules:
You can always select an RPI that is slower than listed above. These
considerations show how fast modules can be scanned - not how fast an
application can use the data. The RPI is asynchronous to the program scan.
Other factors, such as program execution duration, affect I/O throughput.
Determining When the
Controller Updates I/O
The controller continually scans the control logic. One scan is the time it
takes the controller to execute the logic once. Input data transfers to the
controller and output data transfers to output modules are asynchronous to
the logic scan.
Type of Module: Considerations:
digital and analog
(any mix)
•
1-4 modules can be scanned in 1.0 ms
•
5-16 modules can be scanned in 1.5 ms
•
some input modules have a fixed 8.0 ms filter, so selecting a faster
RPI has no affect
specialty
•
“full-sized” 1769-SDN modules add 1.5 ms per module
•
1769-HSC modules add 0.5 ms per module
TIP: If you need to ensure that the I/O values being used during logic
execution are from one moment in time (such as at the beginning of a ladder
program), use the Synchronous Copy instruction (CPS) to buffer I/O data.
Refer to the Logix5000 Controllers Common Procedures Programming
Manual , publication number 1756-PM001 for examples of I/O buffering or
to the Logix5000 Controllers General Instruction Set Reference Manual ,
publication number 1756-RM003 for information on the CPS instruction.