CS302 - Digital Logic Design
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Q & A
Short Question & Answers
Q1. What is difference b/w BCD to decimal decoder and binary 4-to-16 bit decoder?
The operation of the BCD-to-Decimal Decoder is the same as a Binary 4-to-16 decoder, the only difference being that the BCD-to-Decimal Decoder has ten output pins instead of sixteen and the input is a valid BCD number.
Q2. Explain major use of decoder circuits?
Decoders have two major uses in Computer Systems.

Selection of Peripheral Devices:
Computers have different internal and external devices like the Hard Disk, CD Drive, Modem, Printer etc. Each of these different devices is selected by specifying different codes. A decoder similar to the Electronic Door Lock/Unlock circuit is used to uniquely select or deselect the appropriate devices.

Instruction Decoder:
Computer programs are based on instructions which are decode by the Computer Hardware and implemented. These instruction codes are decoded by an Instruction Decoder to generate signals that control different logic circuits like the ALU and memory to perform these operations.
Q3: PALS comes in different configurations and are identified by a unique number, identify parts of this number?
PALs come in different configurations they are identified by unique number. The numbers begin with the prefix PAL followed by two digits that indicate the number of inputs followed by a letter L active-low, H active-high or P programmable polarity followed by a single or two digits that indicate the number of outputs. In addition to the standard number there may be suffixes which specify the speed, package type and temperature range
Q4: Explain BCD to Decimal Decoder.
The operation of the BCD-to-Decimal Decoder is the same as a Binary 4-to-16 decoder, the only difference being that the BCD-to-Decimal Decoder has ten output pins instead of sixteen and the input is a valid BCD number. Thus invalid BCD codes 1010, 1011, 1100, 1101, 1110 and 1111 applied at the input of the Decoder do not activate any of the ten outputs.
Q5: What is meant by ABEL?
ABEL which is an acronym for Advanced Boolean Expression Language is hardware description language used for implementing logic designs using PLDs.
Q6: Why preferable to use another method than 5-variable K-Map?
Karnuagh map method becomes difficult to manage when numbers of variables exceed 4. In both the Karnaugh maps, finding the redundant terms is not very obvious. The Quine-McCluskey approach of simplifying Boolean expression is based on an exhaustive search where each minterm is compared with every other minterm in order to remove single variables.
Q7: Uses of Demultiplexer?
Demultiplexer is used to connect a single source to multiple destinations. One use of the Demultiplexer is at the output of the ALU circuit. The output of the ALU has to be stored in one of the multiple registers or storage units. The Data input of the Demultiplexer is connected to the output of the ALU. Each output of the Demultiplexer is connected to each of the multiple registers. By selecting the appropriate output data from the ALU is routed to the appropriate register for storage. The second use of the Demultiplexer is the reconstruction of Parallel Data from the incoming serial data stream. Serial data arrives at the Data input of the Demultiplexer at fixed time intervals. A counter attached to the Select inputs of the Demultiplexer routes the incoming serial bits to successive outputs where each bit is stored. When all the bits have been stored, data can be read out in parallel.
Q8: Define Sequential Circuit.
Digital circuits that generate a new output on the basis of some previously stored information and the new input are known as Sequential circuits. Digital circuits that use memory elements for their operation are known as Sequential circuits.
Q9: How a circuit with multiple outputs is shown in truth table?
Circuits having multiple outputs are represented by multiple function tables one for each output or a single function table having multiple output columns. The example of a BCD to 7-Segment Decoder circuit which has 4 inputs and 7 outputs is considered to explain functions having multiple outputs.
Q10: How decoder is used as demultiplexer.
A Demultiplexer is available as a Decoder/Demultiplexer chip which can be configured to operate as a Demultiplexer or a Decoder.
Q11: Two bit comparator? Explain by at least one example.
A 2-bit Comparator circuit compares two 2-bit numbers A and B. The comparator circuit has three outputs. It sets the A>B output to 1 if A>B. It sets the A=B output to 1 if A=B and sets A<B output to 1 if A < B.
Q12: Explain PLDs.
Programmable Logic Devices are used in many applications to replace the Logic gates and MSI chips. PLDs save circuit space and reduce and save the cost of components in a Digital Circuit. PLDS consists of Arrays of AND gates and OR gates that can be programmed to perform specific functions.
Course Instructor

Dr. Waseem Ikram
PhD Electrical Engineering
University of Manchester

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