The keyboard circuit of Figure 9-16 is designed to accept a three digit decimal number. What would happen if four digit keys were activated (e.g., 3095)? Design the necessary logic to be added to this circuit so that after three digits have been entered, any additional digits will be ignored until the CLEAR key is depressed. In other words, if 3095 is entered on the keyboard, the output registers will display 309 and will ignore the 5 and any subsequent digits until the circuit is cleared.

Figure 9-16

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Digit keys +5V 1 k For switch bounce ▶T OS Q Q 20 ms Pull-up resistor for each input 74LS147 decimal- to-BCD encoder J CLEAR CLK ā MSB ō₂ CLR +5 V ā LSB X k(3 J CLA To CLR inputs of Qo-Q₁1 PRE CLK D D D D Chol CLK D To decoder/drivers and displays 0₁. D CLK CLK CLK 9 CLK CLK Q₂ CLK CLK CLK CLK D CLK Q₂ a CLK BCD code for MSD Second MSD LSD