The pitfalls of instrument compatibility

In military and aerospace applications test systems are expected to have a service life of more than 20 years. Over the life of the test system individual test instruments may become obsolete or may require upgrading due to changes in TPS requirements. Any changes to these legacy test instruments may cause problems with the deployed Test Program Set (TPS). Most test systems are designed with an instrument wrapper layer to isolate the TPS developer from the instrument driver Application Programming Interface (API). This wrapper layer will minimize the changes to the TPS due to changes in individual test instruments. This paper discusses some of the issues that may be encountered when either replacing obsolete instrumentation or upgrading instrumentation in a test system. Some of the issues are easy to discover and understand such as minor differences in the instrument specifications between the legacy test instrument and the replacement instrument. Other issues may not become apparent until attempting to use the replacement instrument within a test program or when upgrading system checkers. For example a test program may fail with a replacement instrument due to signal settling time. This problem can occur when the replacement instrument performs measurements faster than the legacy instrument. The problem can be fixed by either adding a wait in the test program or by adding a trigger delay in the test program or wrapper. The main focus of this paper will be on the problems related to the replacement of analog instruments. This paper will not discuss issues related to differences in the instrument driver API since this can usually be handled in the instrument wrapper layer. The problem discussions will look at classes of problems and site some specific examples in each class. Guidelines will be presented for solving or avoiding the problem. Some of this information may also be useful in the design of future test systems and TPSs to avoid similar issues.