Outstanding Leadership Stan Toler. Narasimha Murthy Ph. D yayavaram yahoo. In bus-oriented systems we need various logic devices like buffers,drivers,encoders ,decoders ,multiplexers and de- multiplexers etc.. Especially ,tri-state logic devices are very much required for the proper functioning of multiplexed bus sytems. For example in a microprocessor or microcontroller we find the data and address busses are multiplexed.
So , there is a need of these logic devices in every microprocessor based system. Tri-state Devices : A logic device which has three states, logic 0,logic 1 and and high impedance state is known as a tri-state device.
The third state is also called Enable. When this line is activated the tristate device functions same way as ordinary logic device. Similarly when the third line is disabled the logic device goes to the high impedance state-as if it were disconnected from the system. In microprocessor or microcontroller based systems ,peripherals are connected in parallel between the address bus and data bus. The microprocessor communicates with one device at a time by enabling the tri-state line of that interfacing device.
Tri-state logic prevents a bus conflict where one device is driving a signal to 1 and another device is driving it to 0 at the same time,which may generate high current through wires.
Buffer : A buffer is a logic circuit that increases or amplifies or boosts the current or power of a logic circuit or device. As it increases the driving capability of a circuit it is also known as a Driver. When the output current of a device is insufficient to drive another device ,a buffer is used to increase the current rating.
This device transfers the data only in one direction,by increasing the driving capability of the Bus. For Ex: 74LS is a unidirectional Octal buffer.
The 74LShas simply 8 tri-state buffers in a single chip. These buffers are divided into two groups. The enabling and disabling of these groups are controlled by 1G and 2G lines, until these lines are enabled ,the output of the drivers remains in the high impedance state. It is also known as line driver or line receiver. The address bus is unidirectional buffer,74LS is used to buffer higher order address bus. It consists of 8-non inverting buffers with try state outputs.
Each one can sink 24 mA and source 15 mA of current. Bi-directional Buffer : This bidirectional buffer is used to increase the driving capability of the data bus. It consists 16 non-inverting buffers, eight for each direction,with tri-state output. The direction of data flow is controlled by the pin DIR.
Each buffer in device can sink 24 mA and 15 mA source of current. The direction of the data flow is controlled by the pin DIR. When DIR is high ,the data flows from 3. Each buffer in this chip can sink a current of 24mA and source a current of 15mA. The logic diagram and the functionl table of LS is shown below.
The decoder is used to decode an address bus to identify an output device. Decoders are also built internal to a memory chip to identify individual memory registers. The decoder is a logic device that identifies each combination of the signals present at its input.
If the input to a decoder has two binary lines ,the decoder will have four output lines. Commonly available decoders are 3-to-8 ,4- to- 16 and 4-to to decode BCD input.
Also some decoders have active low output lines as well as Enable lines. The 74LS decodes one-of-eight lines, based upon the conditions at the three binary select inputs and the three enable inputs. Two active-low and one active-high enable inputs reduce the need for external gates or inverters when expanding. Encoder : The encoder is a logic circuit that provides a code Binary or BCD at the output for every input signal applied to it.
This is the reverse process of the Decoding. Normally digital encoders produce outputs of 2-bit, 3-bit or 4-bit codes depending upon the number of data input lines. An "n-bit" binary encoder has 2n input lines and n-bit output lines with common types that include 4-to-2, 8-to-3 and to-4 line configurations.
The output lines of a digital encoder generate the binary equivalent of the input line whose value is equal to "1" and are available to encode either a decimal or hexadecimal input pattern to typically a binary or B. For example, if we make inputs D1 and D2 HIGH at logic "1" both at the same time, the resulting output is neither at "01" or at "10" but will be at "11" which is an output binary number that is different to the actual input present.
Also, an output code of all logic "0"s can be generated when all of its inputs are at "0" OR when input D0 is equal to one. A simple method to overcome this problem is to "Prioritize" the level of each input pin and if there was more than one input at logic level "1" the actual output code would only correspond to the input with the highest designated priority.
Then this type of digital encoder is known commonly as a Priority Encoder or P-encoder. The TTL 74LS is an 8-to-3 bit priority encoder which has eight active LOW logic "0" inputs and provides a 3-bit code of the highest ranked input at its output. Priority encoders output the highest order input first for example, if input lines "D2", "D3" and "D5" are applied simultaneously the output code would be for input "D5" "" as this has the highest order out of the 3 inputs. Once input "D5" had been removed the next highest output code would be for input "D3" "" , and so on.
The Encoder chip has five output lines. Three are encoding lines and two are output enable indicators. The output lines GS and EO can be used to encode more than eight inputs by cascading these devices. When the encoder is enabled and two or more input signals are activated simultaneously ,it ignores the low priority inputs and encodes the highest priority inputs.
Encoders are commonly used with keyboards. For each key pressed the corresponding binary code is Placed on the data bus. It is also called a transparent latch. The 74LS75 is an example for D flip-flop. The output changes according to 6. The is a positive edge triggered flip-flopwhich is shown below.. A typical example for transparent Latch is 74LS chip. When the microprocessor sends the output data , the data is available on the data bus only for few micro seconds, there after a Latch is used to hold the data for display.
The logic diagram of the IC 74LS is shown below. The IC 74LS is an octal buffer latch which consists of eight D latches with tri-state buffers and require two input signals. Enable G and output control OC The enable is an active high signal connected to the clock input of the flip-flop. When this signal goes low,data is latched from the data bus. So, a Multiplexer has N-inputs and one output only. This is also known as Data selector or simply MUX. For Ex: 74LS is a digital multiplexer with 8-input lines, three select lines and one strobe line and one output.
The logical diagram is shown below. In the diagram A,B and C are the select lines. Based on the value on select lines the corresponding input is selected. For input D0 is selected. Similarly for input D7 is selected.. The strobe line enables the output.
When it is High ,the output is enabled. Multiplexer are used to give multiple inputs to a processor or Microcontroller. It performs the reverse operation of a Mux. In a demux ,the input data can be made available at any one of the output lines.
This would allow for a daisy-chain type configuration. You would need external analog switches. Site Search User. Data converters Data converters forum. Mentions Tags More Cancel. Ask a related question What is a related question? A related question is a question created from another question. When the related question is created, it will be automatically linked to the original question. Ask a new question Ask a new question Cancel. If one needs a sample and hold circuit for the conversion of fast signal into equivalent digital quantities, it has to be externally connected at each of the analog inputs.
Interfacing Digital to Analog Converters: The digital to analog converters convert binary number into their equivalent voltages. The DAC find applications in areas like digitally controlled gains, motors speed controls, programmable gain amplifiers etc. Developed by Therithal info, Chennai. Toggle navigation BrainKart. Posted On : In most of the cases, the PPI is used for interfacing the analog to digital converters with microprocessor.
Issue start of conversion pulse to ADC 3. Serial Port Programming: Serial Communication. Interfacing a Microprocessor to Keyboard. Stepper Motor Interface.
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