Measurement Computing DaqScan 2000 Series - Manual (Page 52)

8-2 Taking Measurements 898095 DaqScan/2000 Series
Triggering
Triggering can be the most critical aspect of acquiring data. DaqScan/2000 Series devices
support a full complement of trigger modes to accommodate a wide variety of applications.
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. DaqScan/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). Applies to DaqScan/2001 and DaqScan/2005 only. 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). The devices 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
that it permits triggering based on more complex situations. For example: to trigger 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, 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 Daq 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.