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National Instruments NI cDAQ-9172 Prescaling, Duplicate Count Prevention

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NI cDAQ-9172 User Guide and Specifications 62 ni.com

Prescaling

Prescaling allows the counter to count a signal that is faster than the

maximum timebase of the counter. The NI cDAQ-9172 chassis offers 8X

and 2X prescaling on each counter (prescaling can be disabled). Each

prescaler consists of a small, simple counter that counts to eight (or two)

and rolls over. This counter can run faster than the larger counters, which

simply count the rollovers of this smaller counter. Thus, the prescaler acts

as a frequency divider on the Source and puts out a frequency that is

one-eighth (or one-half) of what it is accepting.

Figure 42. Prescaling

Prescaling is intended for frequency measurement where the measurement

is made on a continuous, repetitive signal. The prescaling counter cannot

be read; therefore, you cannot determine how many edges have occurred

since the previous rollover. You can use prescaling for event counting

provided it is acceptable to have an error of up to seven (or one). You can

use prescaling when the counter Source is an external signal. Prescaling

is not available if the counter Source is one of the internal timebases

(80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).

Duplicate Count Prevention

Duplicate count prevention (or synchronous counting mode) ensures that a

counter returns correct data in applications that use a slow or non-periodic

external source. Duplicate count prevention applies only to buffered

counter applications such as measuring frequency or period. In such

buffered applications, the counter stores the number of times an external

Source pulses between rising edges on the Gate signal.

External Signal

Counter Value

Prescaler Rollover

(Used as Source

by Counter)

01

Contents

Main USER GUIDE AND SPECIFICATIONS NI cDAQ-9172 Safety Guidelines Safety Guidelines for Hazardous Voltages National Instruments Corporation 3 NI cDAQ-9172 User Guide and Specifications Installing the NI cDAQ-9172 Figure 2 shows the dimensions of the NI cDAQ-9172 chassis. Figure 2. NI cDAQ-9172 with Dimensions in Millimeters (Inches) Mounting the NI cDAQ-9172 NI 9901 Desktop Mounting Kit Mounting the NI 9910 DIN-Rail Kit Mounting the NI 9905 Panel Mount Kit Figure 5. Panel Mount Dimensions and Installation on the NI cDAQ-9172 Setting Up the NI cDAQ-9172 Page Understanding LED Indications Active LED Ready LED Using the NI cDAQ-9172 C Series I/0 Modules Correlated vs. Static DIO Modules cDAQ Module Interface USB-STC2 AI and AO Sample Timing Triggering Modes Analog Input Analog Input Triggering AI Start Trigger Signal Routing AI Start Trigger to an Output Terminal AI Reference Trigger Signal Page Routing AI Reference Trigger Signal to an Output Terminal AI Pause Trigger Signal Analog Input Timing Signals AI Sample Clock Routing AI Sample Clock to an Output Terminal AI Sample Clock Timebase Convert Behavior For Analog Input Modules Scanned Simultaneous Sample-and-Hold Sigma-Delta Slow Sample Rate Modules Getting Started with AI Applications in Software Analog Output Analog Output Data Generation Methods Software-Timed Generations Hardware-Timed Generations Buffered Analog Output Analog Output Triggering Analog Output Timing Signals AO Sample Clock Routing AO Sample Clock to an Output Terminal AO Sample Clock Timebase AO Start Trigger Signal Routing AO Start Trigger Signal to an Output Terminal AO Pause Trigger Signal Page Minimizing Glitches on the Output Signal Getting Started with AO Applications in Software Digital I/O Correlated vs. Static DIO Modules Static DIO Digital Waveform Acquisition (Correlated Input) DI Sample Clock Signal Using an Internal Source Using an External Source Digital Waveform Generation (Correlated Output) Buffered Digital Waveform Generation Page PFI Counters Figure 15. NI cDAQ-9172 Counters Counter Input Applications Counting Edges Single-Point (On-Demand) Edge Counting Buffered (Sample Clock) Edge Counting Controlling the Direction of Counting Pulse-Width Measurement Single Pulse-Width Measurement Buffered Pulse-Width Measurement Period Measurement Single Period Measurement Buffered Period Measurement Semi-Period Measurement Single Semi-Period Measurement Buffered Semi-Period Measurement Frequency Measurement Method 1Measure Low Frequency With One Counter Method 1bMeasure Low Frequency With One Counter (Averaged) Method 2Measure High Frequency With Two Counters Method 3Measure Large Range of Frequencies Using TwoCounters Page Choosing a Method for Measuring Frequency Position Measurement Measurements Using Quadrature Encoders X1 Encoding X2 Encoding X4 Encoding Channel Z Behavior Measurements Using Two Pulse Encoders Two-Signal Edge-Separation Measurement Single Two-Signal Edge-Separation Measurement Buffered Two-Signal Edge-Separation Measurement Counter Output Applications Simple Pulse Generation Single Pulse Generation with Start Trigger Retriggerable Single Pulse Generation Pulse Train Generation Continuous Pulse Train Generation Frequency Generation Using the Frequency Generator Frequency Division Counter Timing Signals Counter n Source Signal Routing a Signal to Counter n Source Routing Counter n Source to an Output Terminal Counter n Gate Signal Routing a Signal to Counter n Gate Counter n Aux Signal Routing a Signal to Counter n Aux Counter n A, Counter n B, and Counter n Z Signals Routing Signals to A, B, and Z Counter Inputs Routing Counter n Z Signal to an Output Terminal Counter n Up_Down Signal Counter n HW Arm Signal Counter n Internal Output and Counter n TC Signals Routing Counter n Internal Output to an Output Terminal Frequency Output Signal Routing Frequency Output to a Terminal Default Counter/Timer Routing Start Trigger Pause Trigger Other Counter Features Cascading Counters Counter Filters Page Prescaling Duplicate Count Prevention Example Application That Works Correctly (No Duplicate Counting) Example Application That Works Incorrectly (Duplicate Counting) Example Application That Prevents Duplicate Count When To Use Duplicate Count Prevention Enabling Duplicate Count Prevention in NI-DAQmx Synchronization Modes 80 MHz Source Mode Other Internal Source Mode External Source Mode Digital Routing and Clock Generation Clock Routing 80 MHz Timebase 20 MHz Timebase 100 kHz Timebase Specifications Analog Input Analog Output Digital Waveform Characteristics (Slots 1 through 4 Only)1 PFI Characteristics (Slots 5 and 6 Only) General-Purpose Counter/Timers (Slots 5 and 6 Only)1 Frequency Generator (Slots 5 and 6 Only) External Digital Triggers (Slots 5 and 6 or with Some AI Modules) Module I/O States Power Requirements Bus Interface Physical Characteristics Safety Standards Environmental Shock and Vibration Electromagnetic Compatibility CE Compliance Waste Electrical and Electronic Equipment (WEEE) Where to Go for Support