Diagnosing a blank MSD in a display multiplexer In a system using 74157 multiplexers, a 7449 decoder/driver, and a 74139 decoder, the most significant digit (MSD) display is blank while the least significant digit (LSD) works. Inputs and outputs on 74157/7449 toggle, 74139 input A toggles, but on 74139 the 0 output stays LOW and the 1 output stays HIGH. Inputs B and EN on 74139 read LOW. What is the likely cause and corrective action?

Introduction / Context:
Troubleshooting multiplexed display systems requires understanding enable logic and active-LOW outputs. The 74139 is a dual 2-to-4 decoder with active-LOW outputs and active-LOW enables. If its outputs do not switch as the select inputs change, the selected display may blank or remain stuck.

Given Data / Assumptions:

• 74139: active-LOW enable (EN = 0 enables) and active-LOW outputs (Yn = 0 when selected).
• Measured: A input toggles; B = 0 and EN = 0 (so the channel is enabled, selecting between Y0/Y1 as A toggles).
• Observed: Y0 stuck LOW, Y1 stuck HIGH regardless of A.

Concept / Approach:

With EN = 0 and B = 0, Y0 and Y1 should alternate: when A = 0, Y0 = 0 and Y1 = 1; when A = 1, Y1 = 0 and Y0 = 1. If Y0 is always 0 and Y1 always 1, either A is not truly reaching the chip (bad connection) or the internal logic is defective. Because the scope already shows A toggling at the pin and EN is reported LOW, a chip fault is likely; however, intermittent or lifted EN connections can mimic the symptom too.

Step-by-Step Solution:

Verification / Alternative check:

Temporarily swap the two sections of the 74139 (if practical) or substitute a known-good 74139; if the fault follows the chip, it confirms device failure. Continuity testing on EN validates the alternate hypothesis.

Why Other Options Are Wrong:

• Definite “replace only” (a) ignores possible EN connection fault.
• “EN bad connection only” (b) ignores the strong chance of device failure.
• “Display bad” (d) is unlikely because the select logic itself is not switching.

Common Pitfalls:

• Forgetting that 74139 outputs are active-LOW; stuck LOW looks like a constant selection.
• Probing upstream nodes only and missing a broken enable line.

Final Answer:

The 74139 is defective and must be replaced; or the EN pin on the 74139 could have a bad connection, which should be checked.