Reading Continuous Time Waveforms - Allen-Bradley 1444-TSCX02-02RB User Manual

Reading Continuous Time Waveforms

This section provides guidance for how to assemble one continuous time
waveform from two overlapped asynchronous
overlapped samples can be read from the module is a function of the
configured sample rate, the responsiveness of the module to data requests
("how busy" the module is), the throughput rate of the network, and the
performance of the personal computer and software.
The efficiency of the data transfer to the personal computer increases with
TWF block size (number of samples). Large TWFs require less overhead such
as establishing and terminating connections and initiating data requests.
Four values are required to implement the function.
Value
Sample Time
TWF Period
Sample Period in Microseconds This value is the SamplePeriodInSecs parameter, included in the TWF Record,
Sample Relative Time
(1) It is not possible to assemble continuous synchronously sampled TWFs.
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
(1)
sampled waveforms. Whether
Description
The sample time is the "TimestampSec" attribute that is included in the TWF
Record. In most cases, this attribute is sufficient. However, if subsecond accuracy is
required then also consider the "TimestampNanaoSec" parameter.
IMPORTANT: The Sample Time alone cannot be used to align overlapped time
waveforms due to:
• If Time Synchronization is enabled (Controller Properties), then the controller
is continually adjusting the modules clock. While in most cases no adjustment
is made, even a millisecond change can affect absolute difference in time
between TWFs measured before and after the clock update.
• Effects that are associated with moving the data between the DSP, where it is
sampled, to the Auxiliary processor, where the time stamp is applied, can
cause the time of any calculated sample to be off by one or more samples.
See Reading TWF Data on page 332 for instructions on how to calculate the TWF
period.
which is multiplied by 1,000,000.
This value is the RelativeTime parameter that is included in the TWF Record. This
timer runs in micro seconds, based on the clock in the DSP – so is independent of
UDT time that is held by the Auxiliary processor. The value continuously
increments from 0 to 16777215, then wraps back to 0. Since the module cannot
sample at this rate (1 MHz), it can be used to calculate a unique time for each
sample of a TWF.
CIP Objects Appendix A
329