Logic encoders are integrated circuits (ICs) that generate coded outputs from numeric input lines. Inputs can be obtained from a measuring system that consists of a light source, code disc, opto-electronic scanner, reticle, and opto-array. When light shines through the code disc, dark spots cover combinations of slashes on the reticle. This interrupts the beam of light on the phototransistor and converts the code on the disc into electronic signals, which are then amplified and converted. Common outputs for logic encoders include binary code, binary coded decimal (BCD), decimal-to-BCD, and octal-to-binary. Input and output lines are available in increments of two, and range from two to 16 lines for inputs and two to eight lines for outputs. With priority encoders, a priority is assigned to each input so that when two or more inputs are active simultaneously, the input with the highest priority is represented on the output.
Logic encoders vary in terms of supply voltage, propagation delay, power dissipation, operating current, and operating temperature. Supply voltages range from - 5 V to 5 V and include intermediate voltages such as -4.5 V, -3.3 V, -3 V, 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3 V, 3.3 V, and 3.6 V. The propagation delay is the time interval between the application of an input signal and the occurrence of the corresponding output. Like other ICs, logic encoders require a minimum current for active operation and feature a range of operating temperatures. Some digital comparators are radiation-tolerant. Others provide protection against electrostatic discharge (ESD).
Selecting logic encoders requires an analysis of logic families. Transistor-transistor logic (TTL) and related technologies such as Fairchild advanced Schottky TTL (FAST) use transistors as digital switches. By contrast, emitter coupled logic (ECL) uses transistors to steer current through gates that compute logical functions. Another logic family, complementary metal-oxide semiconductor (CMOS), uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. Logic families for logic encoders include cross-bar switch technology (CBT), Gallium arsenide (GaAs), integrated injection logic (I2L) and silicon on sapphire (SOS). Gunning with transceiver logic (GTL) and gunning with transceiver logic plus (GTLP) are also available.
Logic encoders are available in a variety of IC package types and with different numbers of pins and multiplexers. Basic IC package types for logic encoders include ball grid array (BGA), quad flat package (QFP), single in-line package (SIP), and dual in-line package (DIP). Many packaging variants are available. For example, BGA variants include plastic-ball grid array (PBGA) and tape-ball grid array (TBGA). QFP variants include low-profile quad flat package (LQFP) and thin quad flat package (TQFP). DIPs are available in either ceramic (CDIP) or plastic (PDIP). Other IC package types include small outline package (SOP), thin small outline package (TSOP), and shrink small outline package (SSOP).