Use the USB-6423 block diagram to learn more about the different analog input, analog output, and digital I/O subsystems of the USB-6423 work together.

Figure 1. USB-6423 Block Diagram


Analog Input Block Diagram

Figure 2. USB-6423 Analog Input Block Diagram


The main blocks featured in the USB-6423 analog input circuitry are as follows:

  • I/O Connector—Provides connectivity for analog input signals to the USB-6423. The proper way to connect analog input signals depends on the analog input ground-reference settings.
  • Multiplexers (Mux)—Route one AI channel at a time to the ADC through the NI-PGIA. Each USB-6423 has one ADC.
  • Ground-Reference Settings—Selects between differential, referenced single-ended, and non-referenced single-ended input modes. Each AI channel can use a different mode.
  • NI Programmable Gain Instrumentation Amplifier (NI-PGIA)—Can amplify or attenuate an AI signal to ensure that you use the maximum resolution of the ADC. The NI-PGIA is a measurement- and instrument-class amplifier that minimizes settling times for all input ranges. The USB-6423 uses the NI-PGIA to deliver high accuracy even when sampling multiple channels with small input ranges at fast rates. The USB-6423 can sample channels in any order. You can individually program each channel in a sample with a different input range.
  • Analog-to-Digital Converter (ADC) —Digitizes the AI signal by converting the analog voltage into a digital number.
  • AI FIFO—A large first-in-first-out (FIFO) buffer that holds data during AI acquisitions to ensure that no data is lost. The USB-6423 can perform both single and multiple A/D conversions of a fixed or infinite number of samples. The USB-6423 can handle multiple A/D conversion operations with direct memory access (DMA) or programmed I/O. You can stream the AI data through USB at full rate.

    • Analog Output Block Diagram

    The USB-6423 has four voltage output channels capable of either software-timed single-point updates or hardware-timed waveform generation.

    Figure 3. USB-6423 Analog Output Block Diagram


    The main blocks featured in the USB-6423 analog output circuitry are as follows:

  • DACs—Digital-to-analog converters (DACs) convert digital codes to analog voltages.
  • AO FIFO—The AO FIFO enables analog output waveform generation. It is a first-in-first-out (FIFO) memory buffer between the computer and the DACs. It allows you to either stream a waveform with new data continuously provided from USB or download an entire waveform to the USB-6423 where it can be regenerated entirely from the onboard buffer..
  • AO Sample Clock—The AO Sample Clock signal reads a sample from the DAC FIFO and generates the AO voltage.

    • Digital I/O Block Diagram

    The USB-6423 has 16 bidirectional digital I/O signals that are grouped together in software as a single port referred to as Port 0.

    These signals can function as digital I/O as well as counter, timer, or triggering I/O. When used as counter, timer, or triggering I/O, the lines are referred to as Programmable Function Interface (PFI) lines. The digital I/O lines on the USB-6423 support the following features:

    • Direction and function of each terminal individually controllable
    • Static digital input and output
    • High-speed digital waveform generation
    • High-speed digital waveform acquisition
    • Digital input change detection trigger/interrupt
    • Timing input signal for analog input, analog output, digital input, digital output, or counter/timer functions
    • Timing output signal from analog input, analog output, digital input, digital output, or counter/timer functions
    • Shared I/O voltage logic family selection: 1.8 V, 2.5 V, 3.3 V, or 5 V (shared for all lines)

    The following figure shows the circuitry of one digital I/O line. Each digital I/O line is similar.

    Figure 4. USB-6423 Digital I/O Block Diagram


    In software, these channels are referred to as port0/line0:15 when used as digital I/O and PFI 0:15 when used for other purposes, such as timing I/O.