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  1. 1. 1. What are tri-state devices and why they are essential in a bus oriented system? Electronic logic uses just two states, high and low voltage, or "1" and "0". The output of agate will always be at one value or the other. This is convenient when only a single output is usedto drive a signal. In some cases, it is useful to have two or more outputs driving the same signalline. However, if two outputs are linked together, if they have different outputs, there is likely to bedamage to the outputs and the level on the line will be un-predictable. A tri-state output has thesame high and low levels as standard logic outputs but it has third state, namely high impedance(Z). A high impedance state means that the output is not transferred to the line so effectively; theoutput is simply turned off. Another logic gate can now drive the line and the level is entirelypredictable. Numerous outputs can now drive a single line as long as only on is turned on at anytime.A device which has one input, one enable and one output line is called tri-state device, if twodifferent devices wants a common bus then tri-state device are essential to assign bus to any oneof them in bus orientation system.In a multiplexed bus system, many devices are connected to a common bus. If 2 or more devicesattempt to use the bus at the same time, then data will be lost. Thus only one device must beallowed to use the bus at a time. One method is to connect the devices through tri-state Devices,which when disabled will effectively disconnect devices from the bus.2. Explain priority encoderThe Priority Encoder is another type of combinational circuit similar to a binary encoder, exceptthat it generates an output code based on the highest prioritized input. Priority encoders are usedextensively in digital and computer systems as microprocessor interrupt controllers where theydetect the highest priority input.Priority encoders are available in standard IC form and the TTL 74LS148 is an 8-to-3 bit priorityencoder which has eight active LOW (logic "0") inputs and provides a 3-bit code of the highestranked input at its output. Priority encoders output the highest order input first for example, ifinput lines "D2", "D3" and "D5" are applied simultaneously the output code would be for input"D5" ("101") as this has the highest order out of the 3 inputs. Once input "D5" had been removedthe next highest output code would be for input "D3" ("011"), and so on.The truth table for a 8-to-3 bit Binary Outputpriority encoder is given as:Inputs
  2. 2. D7 D6 D5 D4 D3 D2 D1 D0 Q2 Q1 Q00 0 0 0 0 0 0 1 0 0 00 0 0 0 0 0 1 X 0 0 10 0 0 0 0 1 X X 0 1 00 0 0 0 1 X X X 0 1 10 0 0 1 X X X X 1 0 00 0 1 X X X X X 1 0 10 1 X X X X X X 1 1 01 X X X X X X X 1 1 1Then the final Boolean expression for the priority encoder including the zero inputs is defined as:3. Differentiate between static RAM and dynamic RAM STATIC RAM DYNAMIC RAM 1.Static ram use a number of transistor to 1. Dynamic ram use storage cell made of store single bit. transistor and and capacitor. 2. SRAM store data till power of the computer 2. DRAM refreshed there data every is switched on instance of time and retain the data for a very short span of time, even after the power is off 3. Static Ram is very costly 3. Dynamic Ram is cheaper. 4. Static Ram is faster the Dynamic Ram 4. while dynamic ram is slower then static 5. Static Ram is used in L1 and L2 cache. 5. Dynamic Ram is used in system RAM4. Describe the operation of octal to binary encoder Octal-to-Binary take 8 inputs and provides 3 outputs, thus doing the opposite of what the 3-to-8 decoderdoes. At any one time, only one input line has a value of 1. The figure below shows the truth table of anOctal-to-binary encoder.
  3. 3. TRUTH TABLE I0 I1 I2 I3 I4 I5 I6 I7 Y2 Y1 Y01 0 0 0 0 0 0 0 0 0 00 1 0 0 0 0 0 0 0 0 10 0 1 0 0 0 0 0 0 1 00 0 0 1 0 0 0 0 0 1 10 0 0 0 1 0 0 0 1 0 00 0 0 0 0 1 0 0 1 0 10 0 0 0 0 0 1 0 1 1 00 0 0 0 0 0 0 1 1 1 1 For an 8-to-3 binary encoder with inputs I0-I7 the logic expressions of the outputs Y0-Y2 are:Y1= I2 + I3 + I6 + I7 Y0 = I1 + I3 + I5 + I7Y2 = I4 + I5 + I6 +I7 CRICUIT 5.Differentiate between RAM and ROM memories. RAM ROM 1. RAM is Random Access Memory. 1. ROM is Read Only Memory 2. RAM is the memory available for the operating 2. ROM is the memory that comes with your system, programs and processes to use when computer that is pre-written to hold the the computer is running instructions for booting-up the computer 3. RAM requires a flow of electricity to retain data 3. ROM will retain data without the flow of 4. RAM is a type of volatile memory electricity 5. There are different types of RAM, including 4. ROM is a type of non- volatile memory DRAM (Dynamic Random Access Memory) and 5. There are different types of ROM, including SRAM (Static Random Access Memory). PROM (programmable read-only memory) that is manufactured as blank memory (e.g. a CD- ROM) and EPROM (erasable programmable read-only memory). 6. What is a tri-state buffer? Why we need it? An inverter is called a NOT gate, and it looks like:
  4. 4. The inverter is a triangle, followed by a circle/bubble. That circle sometimes appears by itself, and means negation The buffer is logic circuit that applies the current or power. It has one input line and one output line. The logic output of the buffer is same as its input. A tri-state buffer is a device that allows you to control when an output signal makes it to the bus. When the tri-state buffers control bit is active, the input of the device makes it to the output. This is when the "valve" is open. When its not active, the output of the device is Z, which is high-impedance or, equivalently, nothing. This is when the "valve" is closed, and no electrical signal is allowed to pass to the outputA tri-state buffer is a useful device that allows us to control when current passes through the device, and when it doesnt. A tri-state buffer has two inputs: a data input x and a control input c. The control input acts like a valve. When the control input is active, the output is the input. That is, it behaves just like a normal buffer. The "valve" is closed. When the control input is not active, the output is "Z". The "valve" is open, and no electrical current flows through. Thus, even if x is 0 or 1, that value does not flow7. Describe operation of 4:2 priority encoders A 4-bit priority encoder (also sometimes called a priority decoder). This circuit basically converts the 4-bit input into a binary representation. If the input n is active, all lower inputs (n-1 .. 0) are ignored: x3 x2 x1 x0 y1 y0 1 X X X 1 1 0 1 X X 1 0 0 0 1 X 0 1 0 0 0 X 0 0 The circuit operation is simple. Each output is driven by an OR-gate which is connected to the NAND-INV outputs of the corresponding input lines. The NAND gate of each stage receives its input bit, as well as the NAND gate outputs of all higher priority stages. This structure implies that an active input on stage n effectively disables all lower stages n-1.. 0.
  5. 5. 8. Give classification of memories.Memrory is divided into 2 patrs1.RAM2.ROM1.ROM: This type of memory holds information permanently written on it. Usually it is callednon-volatile memory because it retains the information when the power is turned off. Thisis an important feature of Read Only Memory. ROM is used to store computer startupinstructions. The instruction is pre-programmed by the manufacturer; means you can’tmodify or write back on ROM. Rom is divide into 3 partsPROM (Programmable read-only memory)EPROM (Erasable programmable read-only memory)EEPROM(Electrically Erasable programmable read-only memory)2.RAM: This is a type of memory serves as main memory of a computer. It temporarilystores copy of information and files loaded from a computer hard drive that are required bya processor. After finishing the modification or change you made on the loaded file, it mustbe saved back to the hard drive before the power is turned off. If power is switched offwithout saving, you won’t retrieve the change made on the original file.RAM IS DIVIDEDINTO 2 Parts1. SRAM: .Static ram use a number of transistor to store single bit. SRAM store data till power of the computer is switched onStatic Ram is very costly Static Ram is faster the Dynamic Ram DRAM: Dynamic ram use storage cell made of transistor and and capacitor. DRAM refreshed there data every instance of time and retain the data for a very short span of time, even after the power is off Dynamic Ram is cheaper. while dynamic ram is slower then static
  6. 6. 9.What is a latch? Describe its functions? Latch – a memory element that does not have a clock input. State changes occur in response to a change in data input. Latches are commonly used in asynchronous sequential switching circuits10. What is a decoder? Explain the working of a decoderA decoder is a combinational logic circuit that converts the binary code data at its input into one of anumber of different output lines one at a time producing an equivalent decimal code at its outputBinary Decoders: have inputs of 2-bit, 3-bit or 4-bit codes depending upon the number of data inputlines, and a n-bit decoder has 2n output lines. Therefore, if it receives n inputs (usually grouped as abinary or Boolean number) it activates one and only one of its 2n outputs based on that input with allother outputs deactivated. A decoder’s output code normally has more bits than its input code andpractical binary decoder circuits include 2-to-4, 3-to-8 and 4-to-16 line configurations. A binarydecoder converts coded inputs into coded outputs, where the input and output codes are differentand decoders are available to "decode" either a Binary or BCD (8421 code) input pattern to typically aDecimal output code. Commonly available BCD-to-Decimal decoders include the TTL 7442 or theCMOS 4028. An example of a 2-to-4 line decoder along with its truth table is given below. It consistsof an array of four NAND gates, one of which is selected for each combination of the input signals Aand B.Memory Address Decoder: Binary Decoders are most often used in more complex digital systems toaccess a particular memory location based on an "address" produced by a computing device. Inmodern microprocessor systems the amount of memory required can be quite high and is generallymore than one single memory chip alone. One method of overcoming this problem is to connect lotsof individual memory chips together and to read the data on a common "Data Bus". In order toprevent the data being "read" from each memory chip at the same time, each memory chip isselected individually one at time and this process is known as Address Decoding. In this application,the address represents the coded data input, and the outputs are the particular memory element selectsignals. Each memory chip has an input called Chip Select or CS which is used by the MPU to select theappropriate memory chip and a logic "1" on this input selects the device and a logic "0" on the input de-selects it. By selecting or de-selecting each chip, allows us to select the correct memory device for aparticular address and when we specify a particular memory address, the corresponding memory locationexists ONLY in one of the chips