When an output I/O flag is attached to a net that is also attached to an input, control or bidirectional port on a component or assembly, then that flag is considered to be an internal output flag. Otherwise, the I/O flag is considered to be a terminal output flag (since the signal flow terminates at that flag).
In the example below, OUT1 is a terminal output flag (since its net is not connected to any other input ports), whereas OUT2 is an internal output flag (since the net is also connected to an input of object B).
How are Internal and Terminal Output Flags Interpreted?
The type of test location (internal or terminal) is often considered by the diagnostics when choosing one test over another. Let's say that an Operational Test were to be defined for each of the two output flags in the design depicted above. The order in which these tests are used by the diagnostics would depend upon the selected Diagnostic Algorithm. For example, if a detection algorithm were to always give the highest priority to tests that are least likely to fail, then the test at OUT2 (which only detects failures to object A) would be performed before the test at OUT1 (which would fail if either A or B were to malfunction). On the other hand, a detection algorithm that privileges tests that are most likely to detect a failure would perform only the test at OUT1. If that test were to pass, then both A and B would be proven good and no further testing would be necessary. If the test at OUT1 were to fail, however, then the test at the other location may be used during fault isolation. There are times, however, when it is desirable to achieve as much isolation as possible during fault isolation, yet still reduce the number of tests required to detect a failure. This can be achieved by separating tests into two or more Test Candidate Groupings, based on whether the test is performed at an internal or terminal output flag. Internal tests would be selected before terminal tests yet, within each grouping, the test that is most likely to fail would be selected first.
