Method of in-circuit testing

Claim 1

For performing a test on an electronic circuit comprising a common bus and a plurality of bus devices that are operable by application of enabling and disabling signals thereto to assume, when they are operating properly, enabled and disabled states, respectively, such that they drive the bus voltage to an assertion voltage value when at least one of the bus devices is in its asserted state, and such that the bus voltage assumes a de-assertion voltage value when all of the bus devices are in their disabled states, the purpose of the test being to determine which of the bus devices is driving the bus to an assertion voltage level even though all of the bus devices have had disabling signals applied thereto, a test apparatus comprising: A. driver means operable by application of control signals thereto selectively to apply enabling and disabling signals to the bus devices; B. a programmable back-driving source adapted for application thereto of programming signals representing a current limit, for driving the bus to the de-assertion voltage if the resultant current flow in the bus is less than the current limit represented by the programming signals but limiting its output current to that current limit; C. sensor means for sensing the quiescent bus current and providing an indication thereof; D. variation means for determining whether the bus current has departed from the quiescent bus current by more than a predetermined amount and providing an indication thereof; E. control means, responsive to the sensor means and the variation means and connected for application of control signals to the driver means and programming signals to the back-driving source, for operating the driver means to apply disabling signals to all of the bus devices so that all properly operating bus devices are disabled, operating the back-driving source to back-drive the bus to the de-assertion voltage level while all properly operating devices remain disabled, operating the sensor means to measure the quiescent current that results from the back-driving, selecting each bus device in succession, operating the driver means to apply enabling signals to the selected bus device individually while the other bus devices that are operating properly remain disabled, applying programming signals to the back-driving source to cause it to drive the bus with a current limit greater than the quiescent current but less than the sum of the quiescent current and the assertion-current capacity of the selected bus device, and providing an indication that the selected bus device is defective if the variation means indicates that the bus current has departed from the quiescent current by more than the predetermined amount.