User Manual

Rockwell Automation Publication 1756-UM005B-EN-P - January 2013 101
Appendix A
Data Storage
You can store module data in controller tags by using an Event task.
Timing Relationships
This section describes the timing relationship between the module’s RPI, RTS,
and an Event task with a Module Input Data State Change trigger. This type of
Event task monitors a module’s input data and is triggered each time the input
data changes.
The following information applies when the 1756-IF4FXOF2F module is in the
same chassis as its owner-controller:
•RTS—The rate at which the module retrieves new input data samples
from its channels. With each RTS, the module scans all channels. Because
the module cannot scan all channels simultaneously, there is an
approximate 1 µs time gap to scan all channels. Once the module
completes scanning all channels, it sends updated data across the
backplane, and the Event task is triggered.
•RPI—The rate at which the module produces the data currently stored in
on-board memory and receives data from the controller. The module’s
outputs are always updated at the RPI rate regardless of the RTS value.
Regardless of the RPI and RTS rates, the module sends new input channel data to
the controller only at the RTS rate, and the Event task is triggered only when the
controller receives new data.
Topic Page
Timing Relationships 101
Choose a Communication Format 102
Use an Event Task to Store Module Data 104
Module Configuration Result
RPI <
RTS The module produces data at the RPI rate, but produces new data only at the RTS rate. In
this scenario, RPI data is the same data produced by the previous RTS. See Figure 2 on
page 20.
The Event task is triggered only when new data is produced at the RTS rate.
EXAMPLE: If the RPI = 8 ms and the RTS = 11 ms, the module produces data every 8 ms,
but produces new data only every 11 ms. The Event task is triggered every 11 ms.
RPI ≥ RTS The module produces only new data and always produces data at the RTS rate.
The Event task is triggered only when new data is produced at the RTS rate.