Chapter 8 Counters© National Instruments Corporation 8-31 NI USB-621x User ManualCounter TriggeringCounters support three different triggering actions:• Arm Start Trigger—To begin any counter input or output function,you must first enable, or arm, the counter. Software can arm a counteror configure counters to be armed on a hardware signal. Software callsthis hardware signal the Arm Start Trigger. Internally, software routesthe Arm Start Trigger to the Counter n HW Arm input of the counter.For counter output operations, you can use the arm start trigger inaddition to the start and pause triggers. For counter input operations,you can use the arm start trigger to have start trigger-like behavior. Thearm start trigger can be used for synchronizing multiple counter inputand output tasks.When using an arm start trigger, the arm start trigger source is routedto the Counter n HW Arm signal.• Start Trigger—For counter output operations, a start trigger can beconfigured to begin a finite or continuous pulse generation. Once acontinuous generation has triggered, the pulses continue to generateuntil you stop the operation in software. For finite generations, thespecified number of pulses is generated and the generation stops unlessyou use the retriggerable attribute. When you use this attribute,subsequent start triggers cause the generation to restart.When using a start trigger, the start trigger source is routed to theCounter n Gate signal input of the counter.Counter input operations can use the arm start trigger to have starttrigger-like behavior.• Pause Trigger—You can use pause triggers in edge counting andcontinuous pulse generation applications. For edge countingacquisitions, the counter stops counting edges while the externaltrigger signal is low and resumes when the signal goes high or viceversa. For continuous pulse generations, the counter stops generatingpulses while the external trigger signal is low and resumes when thesignal goes high or vice versa.When using a pause trigger, the pause trigger source is routed to theCounter n Gate signal input of the counter.
