MSI Logic Circuits

Refer to the keyboard encoder in figure (a). Sometimes when the 5 key is pressed, the system attached to the keypad does not respond. The 5 input on the 74147 is monitored with a digital storage scope while repeatedly pressing the 5 key, and the waveform in figure (b) is obtained; the P above the trace indicates the points at which the technician pressed the key. What is most likely wrong with the circuit?

A The switches on the 5 key are intermittent; the contacts need to be cleaned or the switch replaced.

B The pull-up resistor connected to the 5 key is bad and should be replaced.

C The common ground connection at the bottom of the 0 key has a bad solder connection; repair the connection.

D The 74147 is intermittent, possibly due to high temperature, and should be replaced.

ANS:A - The switches on the 5 key are intermittent; the contacts need to be cleaned or the switch replaced.

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