美国仪器NI PCIe-6321 X数据采集卡

Simultaneous Sampling

A property of a measurement system in which each input or output channel is digitized or updated at the same instant. You will need simultaneous sampling for applications where the time relationship between inputs is important, such as phase analysis of AC signals.

Because channel-dedicated analog-to-digital converters (ADCs) are used to sample data simultaneously between inputs, the maximum sample rate is applicable to each channel and is not divided by the total number of channels. Devices that don’t have simultaneous sampling capabilities use multiplexing to connect each channel to its ADC.

Interval scanning is a low-cost option to simulate simultaneous sampling for low-frequency signals such as temperature or pressure. This method scans the input channels at one interval and uses a second interval to determine the time before repeating the scan. The input channels are scanned within microseconds, creating the effect of simultaneously sampling the input channels.

Current Flow

A sinking device provides a path for the current to ground and is not responsible for powering the device. Terms used to describe sinking devices include NPN, Open Collector, Normally High, and IEC Negative Logic.

A sourcing device provides the power or a positive potential. Sourcing devices ''push'' the current through the load. Terms used to describe sourcing devices include PNP, Open Emitter, Normally Low, and IEC Positive Logic.

Relays are classified by their number of poles and number of throws. The pole of a relay is the terminal common to every path. Each position that the pole can connect to is called a throw. A relay can be made of n poles and m throws. For example, a single-pole single-throw relay (SPST) has one pole and one throw.

• Form A relays are SPST with a default state of normally open.=
  • Form B relays are SPST with a default state of normally closed.
    • Form C relays are SPDT and break the connection with one throw before making contact with the other (break-before-make)..

     

Digital I/O Watchdogs

NI digital I/O watchdogs are an innovative technology that provides protection against a wide variety of fault conditions:

• Computer crash - total OS crash
  • Application crash - software application ceases to respond
    • Driver crash - device driver ceases to respond
      • PCI bus failure - communications cease to respond

      With watchdogs, the digital outputs go to a safe state when a fault condition is detected and the watchdog timer expires for safe recovery. Watchdogs are important whenever the module is connected to actuators such as pumps, valves, motors, and relays. The digital I/O module monitors the software application and, if it fails to respond within the time limit, automatically sets the output lines to a user-defined safe state. The module remains in the watchdog state until the watchdog timer is disarmed by the application and new I/O values are written, the device is reset, or the computer is restarted.

Buffered Operations and DMA

The National Instruments counter/timers can capture numerous data points without dead times. These types of measurements, called buffered operations, are valuable in applications that range from statistical analysis on production lines to experiments in molecular chemistry. For instance, when you configure a counter for buffered period measurement, data is moved from the counter into a buffer.

Each edge that initiates a measurement also causes a transfer of the count into the buffer. With buffered operations, data is transferred to the computer memory using DMA or interrupts. DMA offers a considerable performance advantage; if your application requires this performance simultaneously on multiple counter/timers, you must know how many DMA channels are available on a particular counter/timer device. For example, if a device contains three DMA channels and eight counter/timers, you can simultaneously perform three high-speed and five lower-speed, interrupt-based, buffered operations.