Omega OMB-DAQSCAN-2000 Series Manuale Utente

 

8-2   Taking Measurements     

898095

 DaqScan/2000 

Series 

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.