Abstract: Processes for diagnosing boundary scan observed data produced pursuant to boundary scan testing of interconnected circuit devices having three-state bidirectional scan cells. A first set of observed data is produced by controlling each device individually to drive the I/O pins of the device pursuant to a set of test patterns and to scan out observed values, while the three-state bidirectional scan cells of the other devices that are in the high impedance state. The observed values for each device are diagnosed by a process that analyzes the observed data and refers to a pin connection list to isolate as to the pins of each device stuck-at faults, shorted pin faults, open faults between pins, and hardwire faults. A second set of observed data is produced by controlling each device in turn to be a driving device for driving its I/O pins to a first logical state, while the remaining devices are controlled to have their scan cells in the high impedance state and to scan out the signals observed on their I/O pins.