Testability implemented in the VAX 6000 model 400

The author describes two test features in the VAX 6000 model 400 processor which help test and diagnose the module in a manufacturing environment. The continuity transistor structure allows an in-circuit tester to detect and diagnose open faults between the chips and the board using analog instrumentation. The observe boundary scan feature is designed to help detect and diagnose faults during full-speed system test. The overhead caused by both of these test features is negligible in terms of additional silicon area, device pins, and signal delay. The test features combined with a well-controlled assembly process and a very robust design have allowed the VAX 6000 model 400 processor to be manufactured economically