Encyclopedia of Electronic Components Volume 2 by Charles Platt

Author:Charles Platt
Language: eng
Format: mobi, epub, pdf
Tags: TECHNOLOGY & ENGINEERING / Robotics
ISBN: 9781449334130
Publisher: Maker Media, Inc
Published: 2014-11-13T05:00:00+00:00

Figure 14-4. The eight possible inputs of an 8-to-3 encoder (at the top of each panel), and the encoded outputs (below). Note that one input of an encoder must always be logic-high. All logic-low inputs are not a valid state.

Unlike ripple counters, where propagation delays can reduce the overall response time of the component, decoders respond within two or three nanoseconds.

Variants

A simple encoder assumes that only one input pin can be logically active at a time. A priority encoder assigns priority to the highest-value input pin if more than one happens to receive an active input. It ignores any lower-value inputs. An example is the 74LS148, which is an 8-to-3 chip.

A few encoders feature three-state outputs (also known as tri-state), in which a high-impedance or “floating” output state is available in addition to the usual high and low logic states. The high-impedance state allows multiple chips to share an output bus, as those that are in high-impedance mode appear to be disconnected. This is useful if two or more encoders are cascaded to handle a larger number of inputs.

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