Redesign the circuit of Problem 9-16 using a 74HC85 magnitude comparator. Add a “copy overflow” feature that will activate an ALARM output if the OPERATE output fails to stop the machine when the requested number of copies is done.

Data from problem 9-16

Figure 9-76 shows the block diagram of a logic circuit used to control the number of copies made by a copy machine. The machine operator selects the number of desired copies by closing one of the selector switches S₁ to S₉. This number is encoded in BCD by the encoder and is sent to a comparator circuit. The operator then hits a momentary contact START switch, which clears the counter and initiates a HIGH OPERATE output that is sent to the machine to signal it to make copies. As the machine makes each copy, a copy pulse is generated and fed to the BCD counter. The counter outputs are continually compared with the switch encoder outputs by the comparator. When the two BCD numbers match, indicating that the desired number of copies has been made, the comparator output X̅ goes LOW; this causes the OPERATE level to return LOW and stop the machine so that no more copies are made. Activating the START switch will cause this process to be repeated. Design the complete logic circuitry for the comparator and control sections of this system.

Figure 9-76

Transcribed Image Text:

S8 S7 S6 S5 SA S3 S₂ S₁ Decimal-0₁ to-BCD encoder +5 V 03 MSB Start Four-bit B comparator IX D MSB C Control logic A CLK BCD counter CLEAR OPERATE 0 1 Copy pulses machine is OFF machine is ON