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|
|Compare States||L||Disable||Logic Low|
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 communication and real-time hardware comparison of acquired response data.