The digital I/O signals are referenced to D GND. You can individually program each line as an input or output.

The following figure shows P0.<4..7> configured for digital input and P0.<0..3> configured for digital output, the switch receiving TTL signals and sensing external device states, and the LED sending TTL signals and driving external devices.

Figure 59. Digital I/O Connections


Notice Exceeding the maximum input voltage ratings, which are listed in the USB-6421 Specifications, can damage the USB-6421 and the computer. NI is not liable for any damage resulting from such signal connections.

For best signal integrity on high-speed digital signals, use twisted pair wiring and twist each signal wire with a D GND wire. Connect the D GND wire to the D GND pin closest to the signal pin. Note that this requires sharing one D GND pin for every two signals.