Digital Logic States
Test engineers can choose from a number of different digital I/O instruments with a range of features for communication and test applications. Beyond the basic capabilities of driving a digital pattern of 1's and 0's, digital instruments often support waveforms that can include some or all of the logic states shown in the following table.
| Logic State | Drive Data | Expected Response | |
| Drive States | 0 | Logic Low | Don’t Care |
| 1 | Logic High | Don’t Care | |
| Z | Disable | Don’t Care | |
| Compare States | L | Disable | Logic Low |
| H | Disable | Logic High | |
| X | Disable | Don’t Care |
The six logic states control the voltage driver and, if supported, the compare engine of the digital tester on a per clock cycle basis. The Drive states specify what stimulus data the tester drives on a particular channel or when to disable the voltage driver (referred to as the tristate or high-impedance state). Compare states indicate the expected response from the DUT. These six logic states make it possible to perform bidirectional communication and real-time hardware comparison of acquired response data.
The NI 655x digital waveform generator/analyzer supports all six logic states shown in the preceding table, allowing the device to perform bidirectional stimulus/response test options with hardware comparison. Other NI digital waveform generator analyzers can perform simultaneous generation and acquisition using 1's and 0's, but they do not support bidirectional operation.
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