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For any NI 6555/6556 acquisition, the outputs of the dual comparators must be combined into a single bit before being sampled and returned to the software. One comparator uses the Acquisition Voltage Low threshold for comparison, and the other comparator uses the Acquisition Voltage High threshold. You can set these thresholds independently and then configure the data interpretation method to determine the signal behavior, based on the context of these thresholds. The following sections describe the data interpretation in more detail.
High or Low is the default data interpretation method, and this method mimics the data sheet performance of most digital semiconductors. Using this setting, the NI 6555/6556 provides some hysteresis for your acquired signal. When the input signal is sampled below Acquisition Voltage Low, a 0 is received. A 1 is not recognized until the acquired signal passes above Acquisition Voltage Low threshold and above Acquisition Voltage High threshold. Conversely, if the acquired signal was last sampled above Acquisition Voltage High (as a 1), the signal is not be sampled as a 0 until the signal is sampled below Acquisition Voltage High and below Acquisition Voltage Low. In short, signals with voltage in the mid-band (between Acquisition Voltage High and Acquisition Voltage Low) are recognized at the last valid logic level (either above Acquisition Voltage High or Acquisition Voltage Low) at which they were sampled.
This data interpretation method returns an indication of whether the acquired signal is between Acquisition Voltage High and Acquisition Voltage Low. Signals sampled between Acquisition Voltage High and Acquisition Voltage Low (in the mid-band, or Invalid) are returned as a 1, while signals sampled either above Acquisition Voltage High or below Acquisition Voltage Low (Valid) are returned as a 0.
For information about configuring data interpretation with NI-HSDIO, refer to Configuring Data Interpretation.