Chapter 2 Analog InputE Series User Manual 2-16 ni.comOutputting the AI Start Trigger SignalYou can configure the PFI 0/AI START TRIG pin to output theai/StartTrigger signal. The output pin reflects the ai/StartTrigger signalregardless of what signal you specify as its source.The output is an active high pulse. Figure 2-5 shows the timing behavior ofthe PFI 0/AI START TRIG pin configured as an output.Figure 2-5. PFI 0/AI START TRIG Timing BehaviorThe PFI 0/AI START TRIG pin is configured as an input by default.When acquisitions use a start trigger without a reference trigger, they areposttrigger acquisitions because data is acquired only after the trigger. Thedevice also uses ai/StartTrigger to initiate pretrigger DAQ operations. Inmost pretrigger applications, a software trigger generates ai/StartTrigger.Refer to the AI Reference Trigger Signal section for a complete descriptionof the use of ai/StartTrigger and ai/ReferenceTrigger in a pretrigger DAQoperation.AI Reference Trigger SignalYou can use the AI Reference Trigger (ai/ReferenceTrigger) signal to stopa measurement acquisition. In Traditional NI-DAQ (Legacy), a referencetrigger is referred to as a stop trigger. To use a reference trigger, specify abuffer of finite size and a number of pretrigger samples (samples that occurbefore the reference trigger). The desired number of posttrigger samples(samples that occur after the reference trigger) is the buffer size minus thenumber of pretrigger samples.Once the acquisition begins, the DAQ device writes samples to the buffer.After the DAQ device captures the specified number of pretrigger samples,the DAQ device begins to look for the reference trigger condition. If thereference trigger condition occurs before the DAQ device captures thespecified number of pretrigger samples, the DAQ device ignores thecondition.t wt w = 50 to 100 ns
