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National Instruments NI 6238, NI 6239 Digital Isolation, Benefits of an Isolated DAQ Device

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Chapter 9 Isolation and Digital Isolators

NI 6238/6239 User Manual 9-2 ni.com

The non-isolated ground is connected to the chassis ground of the PC or

chassis where the device is installed.

Each isolated ground is not connected to the chassis ground of the PC or

chassis. The isolated ground can be at a higher or lower voltage relative to

the non-isolated ground. All analog measurements are made relative to its

isolated ground signal.

Each isolated ground is an input to the NI 6238/6239 device. The user must

connect this ground to the ground of system being measured or controlled.

Refer to the Connecting Analog Current Input Signals section of Chapter 4,

Analog Input, the Connecting Analog Current Output Signals section of

Chapter5, An alog Output, the Connecting Digital I/O Signals section of

Chapter6, Digital Input and Output, and the Connecting PFI Input Signals

section of Chapter8, PFI, for more informati on.

Digital Isolation

The NI62 38/6239 uses digital isolators. Unlike analog isolators, digital

isolators do not introduce any analog error in the measurements taken by

the device. The A/D converter, used for analog input, is on the isolated side

of the device. The analog inputs are digitized before they are sent across the

isolation barrier. Similarly, the D/A converters, used for analog output, are

on the isolated side of the device.

Benefits of an Isolated DAQ Device

With isolation, engineers can safely measure a small current in the presence

of a large common-mode voltage signal. Some advantages of isolation are

as follows:

•Improved rejection—Isolation increases the ability of the

measurement system to reject common-mode voltages when the

common-mode voltage is applied to the isolated ground.

Common-mode voltage is the signal that is present or “common” to

both the positive and negative input of a measurement device, but is not

part of the signal to be measured.

•Improved accuracy—Isolation improves measurement accuracy by

physically preventing ground loops. Ground loops, a common source

of error and noise, are the result of a measurement system having

multiple grounds at different potentials.

Contents

Main Page Important Information Warranty Copyright Trademarks Patents Page Contents Chapter 4 Analog Input Chapter 5 Analog Output Chapter 6 Digital Input and Output Chapter 7 Counters Page Chapter 8 PFI Chapter 9 Isolation and Digital Isolators Chapter 10 Digital Routing and Clock Generation Page About This Manual Conventions Related Documentation NI-DAQ NI-DAQmx for Linux NI-DAQmx Base LabVIEW LabWindows/CVI Measurement Studio ANSI C without NI Application Software .NET Languages without NI Application Software Device Documentation and Specifications Training Courses Technical Support on the Web Getting Started Installing NI-DAQmx Installing Other Software Installing the Hardware Device Pinouts Page DAQ System Overview DAQ Hardware DAQ-STC2 Calibration Circuitry Sensors and Transducers Cables and Accessories Custom Cabling Programming Devices in Software Connector Information I/O Connector Signal Descriptions Table 3-1. I/O Connector Signals (Continued) RTSI Connector Pinout Analog Input Analog Input Circuitry Analog Input Range Connecting Analog Current Input Signals Method 1 (20 mA (20 mA)) 216 = 610 nA Figure 4-2. Analog Current Input Connection Method 1 Method 2 Figure 4-3. Analog Current Input Connection Method 2 Analog Input Ground-Reference Settings + Page Configuring AI Ground-Reference Settings in Software Multichannel Scanning Considerations Use Short High-Quality Cabling Minimize Current Step between Adjacent Channels Avoid Scanning Faster Than Necessary Example 1 Example 2 Analog Input Data Acquisition Methods Software-Timed Acquisitions Hardware-Timed Acquisitions Buffered Non-Buffered Analog Input Triggering Field Wiring Considerations Analog Input Timing Signals Page Page AI Sample Clock Signal Using an Internal Source Using an External Source Routing AI Sample Clock Signal to an Output AI Sample Clock Timebase Signal AI Convert Clock Signal Using an Internal Source Page Page AI Convert Clock Timebase Signal AI Hold Complete Event Signal AI Start Trigger Signal Routing AI Start Trigger to an Output Terminal AI Reference Trigger Signal Routing AI Reference Trigger Signal to an Output AI Pause Trigger Signal Routing AI Pause Trigger Signal to an Output Getting Started with AI Applications in Software Analog Output Analog Output Circuitry Analog Output Data Generation Methods Software-Timed Generations Hardware-Timed Generations Non-Buffered Buffered Analog Output Triggering Connecting Analog Current Output Signals Analog Output Timing Signals AO Start Trigger Signal Routing AO Start Trigger Signal to an Output AO Pause Trigger Signal Page Page AO Sample Clock Timebase Signal Getting Started with AO Applications in Software Digital Input and Output I/O Protection Programmable Power-Up States Connecting Digital I/O Signals Logic Conventions Getting Started with DIO Applications in Software Counters Figure 7-1. MSeries Counters Counter Input Applications Counting Edges Single Point (On-Demand) Edge Counting Buffered (Sample Clock) Edge Counting Non-Cumulative Buffered 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) Figure 7-12. Method 1b Method 2Measure High Frequency with Two Counters Method 3Measure Large Range of Frequencies Using Two Counters Choosing a Method for Measuring Frequency Page 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 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 Pulse Generation for ETS 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 Routing Counter n Gate to an Output Terminal 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 Routing Signals to Counter n HW Arm Input Counter n Internal Output and Counter n TC Signals Routing Counter n Internal Output to an Output Frequency Output Signal Routing Frequency Output to a Terminal Default Counter Terminals Counter Triggering Arm Start Trigger Start Trigger Pause Trigger Other Counter Features Cascading Counters Counter Filters 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 PFI Using PFI Terminals as Timing Input Signals Exporting Timing Output Signals Using PFI Terminals Using PFI Terminals as Static Digital Inputs and Outputs Connecting PFI Input Signals PFI Filters I/O Protection Programmable Power-Up States Connecting Digital I/O Signals Figure 8-5. NI 6238 Digital I/O Connections (DO Source) Page Isolation and Digital Isolators Digital Isolation Benefits of an Isolated DAQ Device Reducing Common-Mode Noise Digital Routing and Clock Generation Clock Routing 80 MHz Timebase 20 MHz Timebase 100 kHz Timebase External Reference Clock 10 MHz Reference Clock Synchronizing Multiple Devices Real-Time System Integration Bus (RTSI) RTSI Connector Pinout Using RTSI as Outputs Using RTSI Terminals as Timing Input Signals RTSI Filters PXI Clock and Trigger Signals PXI_CLK10 PXI Triggers PXI_STAR Trigger PXI_STAR Filters Page Page Bus Interface DMA Controllers PXI Considerations PXI Clock and Trigger Signals PXI and PXI Express Using PXI with CompactPCI Data Transfer Methods Direct Memory Access (DMA) Interrupt Request (IRQ) Programmed I/O Changing Data Transfer Methods between DMA and IRQ Triggering Triggering with a Digital Source Page A Device-Specific Information NI 6238 NI 6238 Pinout Figure A-1. NI6238 Pinout Table A-1. NI6238 Device Default NI-DAQmx Counter/Timer Pins NI 6238 Specifications NI 6238 Accessory and Cabling Options Screw Terminal RTSI NI 6239 Figure A-2. NI6239 Pinout Table A-2. NI6239 Device Default NI-DAQmx Counter/Timer Pins NI 6239 Specifications NI 6239 Accessory and Cabling Options Screw Terminal RTSI Page B Troubleshooting Analog Input Analog Output Counters C Technical Support and Professional Services Page Glossary Numbers/Symbols A Page B C Page D Page E F G H I Page K L M N O P Q R S Page T U V Page Index Symbols Numerics A B C D E F G H I K L M N O P Q R S T