A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion. Unlike a multiplexer that selects one individual data input line and then sends that data to a single output line or switch, Digital Encoder more commonly called a Binary Encoder takes ALL its data inputs one at a time and then converts them into a single encoded output. The encoder is a sensor attached to a rotating object such as a wheel or motor to measure rotation.
By measuring rotation your robot can do things such as determine displacement, velocity, acceleration, or the angle of a rotating sensor. Simply put, an encoder is a sensing device that provides feedback. Encoders convert motion to an electrical signal that can be read by some type of control device in a motion control system, such as a counter or PLC. The encoder sends a feedback signal that can be used to determine position, count, speed, or direction. A multiplexer and a decoder can be used together to allow sharing of a data transmission line by a number of signals.
In the following diagram, the Control input consists of n wires, and there are 2n data inputs and outputs. The Control input determines which of the data inputs is connected to the transmission line. A decoder is a combinational logic circuit that is used to change the code into a set of signals. It is the reverse process of an encoder. Thus, in these cases, there is no single high input line for the circuit to return the corresponding binary number of.
One way is to simply ignore the disallowed input configurations. This approach will work fine as long as we can be sure that the illegal states can never occur.
An implementation of an eight-to-three encoder. The circuit works fine as long as we respect this limitation.
An invalid configuration of inputs generates an erroneous output. Although this may seem strange at first, it is precisely what we want the circuit to do. The truth table for the eight-to-three encoder would be similar to the one shown above for the four-to-two encoder.
However, in the case of the eight-to-three encoder, the full truth table would consist of rows, since it has eight input lines and thus 2 8 , or , possible input configurations. All but eight of these input configurations would be disallowed. As you can see, this truth table is the exact inverse of the table for the three-to-eight decoder that was presented earlier. In this application, the address represents the coded data inputs, and the outputs are the particular memory element select signals.
A typical memory circuit contains a decoder to select which memory element to write, the memory elements themselves, and a mux to select which element to read. As with multiplexers, this most common application of decoders is beyond our current presentation, so instead we will consider a less common, somewhat contrived application. Consider the function of a decoder and the truth table, K-map, or minterm representation of a given function.
Each row in a truth table, each cell in a K-map, or each minterm number in an equation represents a particular combination of inputs. Each output of a decoder is uniquely asserted for a particular combination of inputs. Thus, if the inputs to a given logic function are connected to the inputs of a decoder, and those same inputs are used as K-map input logic variables, then a direct one-to- one mapping is created between the K-map cells and the decoder outputs.
It follows that any given function represented in a truth table or K-map can be directly implemented using a decoder, by simply by OR'ing the decoder outputs that correspond to a truth table row or K-map cell containing a '1' decoder outputs that correspond to K-map cells that contain a zero are simply left unconnected.
In such a circuit, any input combination with a '1' in the corresponding truth table row or K-map cell will drive the output OR gate to a '1', and any input combination with a '0' in the corresponding K-map cell will allow the OR gate to output a '0'.
Note that when a decoder is used to implement a circuit directly from a truth table or K-map, no logic minimization is performed. Using a decoder in this fashion saves time, but usually results in a less efficient implementation here again, a logic synthesizer would remove the inefficiencies before such a circuit was implemented in a programmable device.
Time-multiplexing works if a given signal can carry more traffic than any one message needs. For example, if ten messages each require that 1 Kbit of information be sent every second, and if a communication signal is available that can carry 10 Kbits per second, then time-multiplexing can be used to provide ten 1 Kbit time windows each second, one for each signal.
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