© National Instruments | 5-39NI cDAQ-9181/9184/9188/9191 User ManualPrescalingPrescaling allows the counter to count a signal that is faster than the maximum timebase of thecounter. The cDAQ chassis offers 8X and 2X prescaling on each counter (prescaling can bedisabled). Each prescaler consists of a small, simple counter that counts to eight (or two) androlls over. This counter can run faster than the larger counters, which simply count the rolloversof this smaller counter. Thus, the prescaler acts as a frequency divider on the Source and putsout a frequency that is one-eighth (or one-half) of what it is accepting as shown in Figure 5-37.Figure 5-37. PrescalingPrescaling is intended to be used for frequency measurement where the measurement is made ona continuous, repetitive signal. The prescaling counter cannot be read; therefore, you cannotdetermine how many edges have occurred since the previous rollover. Prescaling can be used forevent counting provided it is acceptable to have an error of up to seven (or one) ticks. Prescalingcan be used when the counter Source is an external signal. Prescaling is not available if thecounter Source is one of the internal timebases (80MHzTimebase, 20MHzTimebase, or100kHzTimebase).Synchronization ModesThe 32-bit counter counts up or down synchronously with the Source signal. The Gate signal andother counter inputs are asynchronous to the Source signal, so the cDAQ chassis synchronizesthese signals before presenting them to the internal counter.Depending on how you configure your chassis, the cDAQ chassis uses one oftwo synchronization methods:• 80 MHz Source Mode• External or Internal Source Less than 20 MHz80 MHz Source ModeIn 80 MHz source mode, the chassis synchronizes signals on the rising edge of the source, andcounts on the third rising edge of the source. Edges are pipelined so no counts are lost, as shownin Figure 5-38.Figure 5-38. 80 MHz Source ModeExternal SignalCounter ValuePrescaler Rollover(Used as Sourceby Counter)0 180 MHz SourceSynchronize Count
