Appendix B Timing Signal InformationNI PXI-4224 User Manual B-4 ni.comThe device also uses AI START TRIG to initiate pretriggered operations.In pretriggered applications, AI START TRIG is generated by a softwaretrigger unless a PFI pin is selected as the source of AI START TRIG. Referto the AI REF TRIG Signal section for a complete description of the use ofAI START TRIG and AI REF TRIG in a pretriggered operation.AI REF TRIG SignalThe AI REF TRIG signal can be input through PFI 0, PXI_Trig<0..5>, orPXI_Star. Refer to Figure B-2 for the relationship of AI REF TRIG to thesequence.As an input, AI REF TRIG is configured in edge-detection mode. You canconfigure the polarity selection for either rising or falling edge. Theselected edge of AI REF TRIG initiates the posttriggered phase of apretriggered sequence. In pretriggered mode, the AI START TRIG signalinitiates the acquisition. The scan counter (SC) indicates the minimumnumber of scans before AI REF TRIG is recognized. After the SCdecrements to zero, it is loaded with the number of posttrigger scans toacquire while the acquisition continues. The device ignores AI REF TRIGif it is asserted prior to the SC decrementing to zero. After the selected edgeof AI REF TRIG is received, the device acquires a fixed number of scansand the acquisition stops. In pretriggered mode, the device acquires databoth before and after receiving AI REF TRIG.As an output, AI REF TRIG reflects the posttrigger in a pretriggeredsequence, even if the acquisition is externally triggered by another PFI.AI REF TRIG is not used in posttriggered DAQ. The output is an activehigh pulse with a pulse width of 50 to 100 ns. This output is set tohigh-impedance at startup.Figures B-5 and B-6 show the input and output timing requirementsfor AI REF TRIG.Figure B-5. AI REF TRIG Input Signal TimingRising-EdgePolarityFalling-EdgePolarityt w = 10 ns minimumt w
