Measurement Computing LogBook Series - Manual (Page 22)

1-12 An Introduction to LogBook 969591 LogBook User’s Manual
Faulty Control. Control systems based on digital outputs or a DBK25 could fail if dependent on a
fast critical response time.
Distorted Outputs. Analog outputs may appear to be "jaggy" or other distortions such as aliasing-
type errors.
Ideally, each output signal is based on one input, resulting in
an accurate output waveform. Factors such as scan rate,
number of output channels and calculated channels can
overload the system, resulting in one output signal for
multiple input scans. The resulting signal deterioration can
increase over time and shows up as a distorted and/or lagging
output signal. Such output errors can resemble aliasing
errors where output signals are distorted from their input
signals because the effective sampling frequency was not
high enough (see figure).
Solutions To Scan Rate Problems
To confirm a suspected timing problem with your acquisition, run the acquisition and then check:
The LogBook Monitor window in LogView for a corresponding error message.
Actual deterioration of outputs as described in the previous section.
To solve timing problems you may need to perform one or more of the following, in order of effectiveness:
Choose a slower scan rate, or change the trigger parameters.
Reduce the number of output channels.
Reduce the number or complexity of calculated channels.
Reduce the number of input channels.
Estimating an Optimum Scan Rate
Note: The scan rate can be measured as a frequency in Hz or kHz or as a period in ms or µs. These
two measures are reciprocals of each other; e.g., 1/100 kHz = 10 µs.
Processing input channels is LogBook’s highest priority; each input channel is collected at 10 µs. After all
the inputs are collected, LogBook performs the necessary calculations and then updates the enabled outputs.
The time to perform calculations and outputs varies with the type of calculation, and this makes it difficult
to predict the exact length of time required. Simple calculations are done much faster than functions for
non-linear thermocouples and RTDs or the use of logical and bitwise calculated channels. Output channels
can take from 100 µs to 300 µs; so for very approximate results, we’ll use 200 µs.
To estimate the maximum scan rate, use the following formula:
approximate scan period = (number of inputs × 10 µs) + (number of outputs × 200 µs)
If only 5 input channels are enabled, the scan period equals 50 µs with a frequency of 20 kHz. If one output
channel is added, the period becomes 250 µs with a frequency of 4 kHz.
After running the acquisition, check the LogBook Monitor screen for error messages.
A list of Software and Hardware-related error codes is included near the end of the manual.
Output Signal Deterioration