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 1s and 0s, 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 (such as a DAQ device) on a per clock cycle basis. Drive states specify what stimulus data the digital tester drives on a particular channel or when to disable the voltage driver (referred to as the tristate or high-impedence state). Compare states indicate the expected response from the device under test. These six logic states make it possible to perform bidirectional comunication and real-time hardware comparison of acquired response data.