Measurement Computing LogBook Series - Manual (Page 21)

LogBook User’s Manual 969591 An Introduction to LogBook 1-11
Triggering and Scan Timing
Reference Note: For information on defining triggering conditions through LogView, refer
to the Acquisition Configuration text and screen shots, which are provided in the LogView
document module.
If data collection is desired only under specific conditions, you can specify appropriate trigger conditions.
By defining a trigger, pre-trigger, and post-trigger, you can collect data surrounding a specific event. This
event can be an absolute time or a defined condition such as a particular analog channel measuring a certain
quantity. If a calculated channel is chosen as the trigger source, you have greater flexibility in defining the
trigger based on multiple inputs and conditional logic. An auto re-arm feature allows many successive
acquisitions to take place automatically, with each acquisition using the same settings.
LogBook can be triggered by several types of sources, including analog and digital triggering, multi-step
triggering, multi-channel triggering, time-of-day triggering, and manual trigger. The manual trigger can be
implemented in the following ways:
With a computer attached, you can trigger LogBook from LogView’s LogBook Monitor window.
Without a computer, you can use a manual trigger switch by attaching it to the TTL trigger input
(pin 25, on P1).
With a logic device you can engage the TTL trigger on P1’s pin 25, as programmed.
Without a computer, you can use the LBK1’s manual trigger button.
LogBook time-of-day clock has 1/256-second resolution for data-logging applications where acquisitions
must be performed at specific times during the day. The time of occurrence of each acquisition and its
trigger are also recorded with the data. The scan-to-scan timing may be set by a fixed-frequency pacer
clock. Or, an external clock can start each scan individually to allow the scan rate to track a variable-speed
event (such as engine revolutions).
Note: Time-stamping is done in 1/256-second units; but the absolute trigger is in 1-second units.
For data acquisition applications that include both fast and slow signals, multiple sample rates can be
configured. In the acquisition setup dialog box, a primary acquisition rate and divisors for up to 3 more
rates can be configured. Using multiple sample rates, fast signals like vibration can be sampled at a high
rate while slow signals like thermocouples are sampled at lower rates, optimizing the system’s storage
capacity.
Scan Rate Limitations
LogBook’s internal clock runs at 100 kHz, and this is the fastest scan rate possible with just one input
channel in the scan list and no outputs. As input, calculated, and output channels are added to the scan list,
the usable scan rate is correspondingly reduced. The system does not automatically compute an optimum
scan rate for you. However, LogView will generate an error message in the LogBook Monitor window if
timing problems occur, and the following sections explain such problems and how to solve them.
External TTL Trigger and Stop Events
An external TTL trigger can repeat before the trigger block completes; extra trigger signals will be ignored.
Likewise, multiple stop signals received before restarting the next scan will be ignored. Such ignored
signals are noted in the LogBook Monitor window as ―Losing Trigger Events‖ and ―Losing Stop Events‖.
Problems Arising From Too Fast a Scan Rate
If the user-specified scan rate does not provide enough time to complete the necessary tasks of the entire
scan list, various problems can occur. Bear in mind that LogBook places the highest priority on reading
input channels—it is primarily a data logger. Also realize that calculated and output channels are based on
input channels and come typically at the end of the scan list. Thus, if the scan rate is too fast and the next
sequence begins before the first is completed, the outputs may suffer. When outputs can’t keep up with the
inputs, possible consequences include:
Missing/late Outputs. The outputs are not updated in a timely fashion and may not represent their
sources in real time (LogBook Monitor error message is ―Outputs Deteriorating‖).
Missed Alarms. Digital alarm outputs may not be initiated soon enough—important alarms might
never go off.