Refer the given figure. The outputs (Q0–Q3) of the memory are always LOW. The address lines (A0–A7) are checked with a logic probe and all are indicating pulse activity, except for A3, which shows a constant HIGH, and A7, which shows a constant LOW; the select lines, are checked and shows pulse activity, while indicates a constant HIGH. What is wrong, and how can the memory be tested to determine whether it is defective or if the external circuitry is defective?

• One of the inputs to the active-LOW select AND gate may be stuck high for some reason; take both select lines LOW and check for pulse activity on the outputs, Q0–Q3. If the outputs now respond, the problem is most likely in the program or circuitry driving the select lines.
• The problem appears to be in the two address lines that never change levels; the problem is probably in the program driving the memory address bus.
• The output buffers are probably defective since they are all tied together; the common input line is most likely stuck LOW. Change the output buffer IC.
• Since no data appears to be getting through to the output buffers, the problem may be in the X decoder; change the X decoder IC.