Measurement Computing DaqBook 2000 Series - Manual (Page 64)

8-2 Taking Measurements 898695 DaqBook/2000 and DaqOEM/2000 Series
Triggering
Triggering can be the most critical aspect of acquiring data. DaqBook/2000 and DaqOEM/2000 Series
devices support a full complement of trigger modes to accommodate a wide variety of applications.
Multi-unit synchronization via the SYNC ports does not support pre-trigger data.
Only post-trigger data can be collected and time correlated between the units.
Hardware Analog Triggering. Many data acquisition products claim analog triggering, but rely on the
PC to take readings and make a decision, which leads to uncertain and potentially long latencies.
DaqBook/2000 and DaqOEM/2000 Series devices use true analog triggering, whereby the trigger level
programmed by the user, sets an analog DAC, which is then compared in hardware to the analog input
level on the selected channel. The result is analog trigger latency that is guaranteed to be less than 5 µs,
significantly shorter than most data acquisition devices. Any analog channel can be selected as the trigger
channel. The user can program both the trigger level, as well as the edge (rising or falling).
Digital Triggering (P1). A separate digital trigger input line is provided, allowing TTL-level triggering,
again with latencies guaranteed to be less than 5 µs. Both the logic levels (1 or 0), as well as the edge
(rising or falling), can be programmed for the discrete digital trigger input. The digital trigger input is
labeled TTLTRG.
Digital Pattern Triggering (P2 and P3
[if available] ). The DaqBooks support digital pattern triggering,
whereby the user can designate any of the digital input ports as the trigger port. The programmed digital
pattern, including the ability to mask or ignore specific bits, is then compared to the actual input until a
match is detected, after which the sequencer begins the scan sequence.
Counter Triggering. Triggering can be programmed to occur when one of the counters reaches, exceeds,
or is within a programmed level. Any of the built-in counter/totalizer channels can be programmed as a
trigger source.
Software-Based Triggering. Software-based triggering differs from the modes described above because
the readings [analog, digital, or counter] are interrogated by the PC to detect the trigger event, not in the
hardware as described above. The advantage of this mode is to permit triggering based on more complex
situations, such as on a specific temperature, which was derived from the acquisition of at least two analog
measurements, plus the calculation of the measured temperature using linearization algorithms.
Normally software-based triggering results in long latencies from the time that a trigger condition is
detected, until the actual capturing of data commences. However, the DaqBook circuitry circumvents this
undesirable phenomenon by use of pre-trigger data. Specifically, when software-based triggering is
employed, and the PC detects that a trigger condition has occurred, (which may be thousands of readings
later than the actual occurrence of the signal), the DaqBook driver automatically looks back to the location
in memory where the actual trigger-causing measurement occurred. The acquired data that is presented to
the user actually begins at that point. The latency in this mode is equal to one scan cycle.