You can choose from a number of different digital I/O instruments with a range of features for communication and test applications.

The primary function of digital instruments is to drive a digital pattern of binary 1s and 0s. Digital instruments might also support waveforms that might include certain parts of the logic states from the following table:

Logic State Drive Data Expected Response
Drive States 0 Logic Low Don’t Care
Drive States 1 Logic High Don’t Care
Drive States Z Disable Don’t Care
Compare States L Disable Logic Low
Compare States H Disable Logic High
Compare States 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. This state is 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.

Only the NI 6547/6548/655x digital waveform generators and analyzers support all six logic states shown in the preceding table, allowing the devices to perform bidirectional stimulus/response test options with hardware comparison. Other digital waveform generators and analyzers can perform simultaneous generation and acquisition using 1's and 0's, but they do not support bidirectional operation.