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Chapter 8 Counters
© National Instruments Corporation 8-15 NI USB-621x User Manual
Table 8-2 summarizes some of the differences in methods of measuring
frequency.
For information about connecting counter signals, refer to the Default
Counter/Timer Pinouts section.
Position Measurement
You can use the counters to perform position measurements with
quadrature encoders or two-pulse encoders. You can measure angular
position with X1, X2, and X4 angular encoders. Linear position can be
measured with two-pulse encoders. You can choose either a single point
(on-demand) position measurement or a buffered (sample clock) position
measurement. You must arm a counter to begin position measurements.
Measurements Using Quadrature Encoders
The counters can perform measurements of quadrature encoders
that use X1, X2, or X4 encoding. A quadrature encoder can have up
to three channels—channels A, B, and Z.
• X1 Encoding—When channel A leads channel B in a quadrature
cycle, the counter increments. When channel B leads channel A in a
quadrature cycle, the counter decrements. The amount of increments
and decrements per cycle depends on the type of encoding—X1, X2,
or X4.
Figure 8-14 shows a quadrature cycle and the resulting increments and
decrements for X1 encoding. When channel A leads channel B, the
increment occurs on the rising edge of channel A. When channel B
leads channel A, the decrement occurs on the falling edge of
channel A.
Table 8-2. Frequency Measurement Method Comparison
Method
Number of
Counters
Used
Number of
Measurements
Returned
Measures High
Frequency
Signals
Accurately
Measures Low
Frequency
Signals
Accurately
1 1 1 Poor Good
1b 1 Many Fair Good
2 1 or 2 1 Good Poor
3 2 1 Good Good
- DAQ M Series 1
- Important Information 3
- Compliance 4
- Chapter 4 6
- Chapter 5 7
- Contents 10
- About This Manual 12
- Related Documentation 13
- LabWindows/CVI 15
- Measurement Studio 15
- Training Courses 17
- Technical Support on the Web 17
- Getting Started 18
- Device Specifications 19
- USB Cable Strain Relief 20
- Mounting the USB-621x 21
- Panel Mounting 22
- Chapter 1 Getting Started 23
- DAQ System Overview 24
- DAQ-STC2 25
- Calibration Circuitry 25
- Signal Conditioning 26
- Cables and Accessories 27
- Chapter 2 DAQ System Overview 28
- Connector and LED Information 29
- +5 V Power 31
- USB Device Fuse Replacement 31
- 0.75A,125V Fuse, Socketed 32
- PWR/ACT LED Indicator 33
- Analog Input 34
- Analog Input Range 35
- Chapter 4 Analog Input 36
- Figure 4-2. NI-PGIA 37
- NI-DAQmx Create 38
- Virtual Channel.vi 38
- Field Wiring Considerations 43
- Analog Input Timing Signals 44
- AI Sample Clock Signal 47
- Using an Internal Source 47
- Using an External Source 47
- Other Timing Requirements 47
- AI Convert Clock Signal 49
- AI Hold Complete Event Signal 52
- AI Start Trigger Signal 53
- Using a Digital Source 53
- Reference Trigger 54
- Pretrigger Samples 54
- Complete Buffer 54
- Posttrigger Samples 54
- AI Pause Trigger Signal 55
- USB-6210/6211/6212 Devices 56
- (NRSE Configuration) 68
- AO Range 73
- Chapter 5 Analog Output 74
- Analog Output Timing Signals 75
- AO Start Trigger Signal 76
- AO Pause Trigger Signal 77
- Pause Trigger 78
- Sample Clock 78
- Digital I/O 81
- Chapter 6 Digital I/O 82
- Chapter 7 PFI 89
- Connecting PFI Input Signals 91
- PFI Filters 91
- Figure 7-4. Filter Example 92
- Table 7-1. Filters 92
- I/O Protection 93
- Programmable Power-Up States 93
- Counters 94
- Counter Input Applications 95
- Pulse-Width Measurement 97
- Period Measurement 99
- Single Period Measurement 99
- Buffered Period Measurement 100
- Semi-Period Measurement 101
- Frequency Measurement 102
- Figure 8-10. Method 1 103
- Figure 8-11. Method 1b 104
- Figure 8-12. Method 2 105
- Figure 8-13. Method 3 106
- Chapter 8 Counters 107
- Position Measurement 108
- Channel Z Behavior 109
- Counter Output Applications 113
- Pulse Train Generation 115
- Frequency Generation 116
- Using the Frequency Generator 116
- Frequency Division 117
- Pulse Generation for ETS 117
- Counter Timing Signals 118
- Counter n Source Signal 119
- Counter n Gate Signal 120
- Counter n Aux Signal 120
- Counter n Up_Down Signal 121
- Counter n HW Arm Signal 121
- Default Counter/Timer Pinouts 123
- Counter Triggering 124
- Other Counter Features 125
- Cascading Counters 126
- Counter Filters 126
- Prescaling 127
- Duplicate Count Prevention 128
- Duplicate Counting) 128
- (Duplicate Counting) 129
- USB-6215/6216/6218 Devices 131
- Digital Isolation 132
- Reducing Common-Mode Noise 133
- Isolated Systems 134
- Non-Isolated Systems 134
- 80 MHz Timebase 135
- 20 MHz Timebase 135
- 100 kHz Timebase 135
- Bus Interface 136
- Chapter 11 Bus Interface 137
- Triggering 138
- Chapter 12 Triggering 139
- Device-Specific Information 140
- USB-6210 141
- USB-6211/6215 143
- USB-6212/6216 Screw Terminal 145
- USB-6216 Mass Termination 147
- TERMINAL 1 148
- TERMINAL 34 148
- TERMINAL 35 148
- TERMINAL 68 148
- NC = No Connect 148
- SCC Accessories 149
- BNC Accessories 150
- Screw Terminal Accessories 150
- USB-6218 152
- Troubleshooting 154
- Channel 0 155
- Channel 1 155
- Convert Period 155
- Sample Period 155
- Analog Output 156
- Technical Support and 157
- Professional Services 157
- Glossary 159
- Numerics 175
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