3Encoders Multiple-input/multiple-output device. Perfoms the inverse function of a Decoder.Outputs ( m ) are less than inputs ( n ).Converts input code words into output code words.input codeoutput codeENCODER
5Binary Encoder 2^n-to-n encoder : 2^n inputs and n outputs. Input code : 1-out-of-2^n.Output code : Binary CodeExample : n=3, 8-to-3 encoder Inputs OutputsI0 I I2 I I4 I5 I I Y0 Y1 YI4Y2I5I6I7
68-to-3 encoder Implementation Simplified implementation: - From the truth table Y2 = I1 + I3 + I5 + I7 Y1 = I2 + I3 + I6 + I7 Y0 = I4 + I5 + I6 + I7Limitations : - I0 has no effect on the output - Only one input can be activatedApplication: Handling multiple devices requests But, no simultaneous requestsEstablishing priorities solve the problem of multiple requestsY0I1I2Y1I3I4I5Y2I6I7
14Three State Buffers/Drivers A buffer/inverter with enable inputBuffer Buffer Inverter Inverter Actice High Enable Actice Low Enable Active High Enable Actice Low EnableThe device behaves like an ordinary buffer/inverter when the enable input is asserted.The ouput is floating ( High Impedance, Hi-Z ) when the enable input is deasserted ( The input is isolated from the output, behaves as if it did not exist)Application: Controlling the access of a single line/bus by multiple devices
15Three-state buffers Output = LOW, HIGH, or Hi-Z. Can tie multiple outputs together, if at most one at a time is driven.
21Three-state transceiver Pairs of 3-state buffers connected in opposite directions between each pair of pins, so data can be transferred in either direction.DIR determines the direction of transfer (A-->B or B-->A)
23Multiplexers (mux)Eg. Put between Processor’s registers and ALUA 16-bit processor where 3-bit field specifies on of 8 registers.The 3-bit field is connected to the select inputs of an 8-input, 16-bit mux.Select one of n sources of data to transmit on a bus.
28DemultiplexersA mux is used to select one of n sources of data to transmit on a bus.A demultiplexer can be used to route the bus data to one of m destinations. Just the inverse of a mux.A binary decoder with an enable input can be used as a demux. Eg. 74x139 can be used as a 2-bit, 4-output demux.