Circuit Function: Decoder and Comparator for Lift Movement

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Circuit Function:
The circuit is a combinational circuit that can be built using basic Boolean gates. No memory
is required for the circuit functioning so it eliminates the need of any flip flops for circuit
design. The combination circuit part is built up using two sub components. A decoder and a
comparator.
Floor Decoder:
Decoder is a circuit that takes in n bits input and decodes it into one of the 2n outputs
depending upon value of the input given. Rest all outputs remain low. For the given
question, the input is a three bit value and hence will need a 3 to 8 decoder. However, not
all values are required for circuit and can be optimized. So only valid combinations are
decoded and given as a single bit output. Output being high means value entered for the
input is valid value else it remains 0.
The floor decoder takes the three bit input. It decodes them and gives a valid high output if
the value of input is one of the valid input values. i.e. 1, 2, 3, 5, 7 and stays 0 if input is 0, 4
or 6. The same unit is used twice, one for each input. Outputs from both units are checked
to be high. This would mean Present state of lift and destination of lift are both valid. This
fulfils the necessary condition for movement of lift from its current position.

Comparator:
Another unit is comparator. Function of comparator is to equate if two set of inputs are
equal. This is achieved using an EXOR gate function built using AND, OR and NOT gates. It
gives an output 1 if inputs are different else stay 0 in case both are equal. It takes in three
bit input and matches if both inputs are equal. If inputs are not equal it gives out a single bit
output as 1. This serves necessary condition that C must not be equal to D.
Combining these two outputs gives the final value whether the lift should move or not.