Image credit: Texas Instruments | Electronic Goldmine
Logic multiplexers are integrated circuits (ICs) that route digital information from multiple sources onto a single line for transmission to a common destination. They maximize the efficiency of communication systems and data networks by combining multiple audio, voice, and data applications onto a single subcarrier. This limits interference and allows signals to share modulators and demodulators. Time division places signals onto a shared line at the correct time. Logic multiplexers accept binary numbers as selector inputs and present the logic level connected to the input line as the output from the data selector. Multi-channel inputs and three-state buffered outputs are available. Devices with two inputs can multiplex up to four data signals. With three addressing inputs, logic multiplexers can handle up to eight signals.
Logic multiplexers are widely used in the telecommunications industry and in other industries that require switching applications. With computers, dynamic memory uses the same address lines for both row and column addressing. Consequently, logic multiplexers are used to first select the row address and then switch to the column address. This scheme allows large amounts of memory to be incorporated into the computer while limiting the number of copper traces required to connect the memory to the rest of the computer’s circuitry.
Specifications and Features
Selecting logic multiplexers requires an analysis of performance specifications and features. Devices differ in terms of the number of inputs, operating current, operating temperature, and minimum supply voltage required for active chip operation. Logic multiplexers also differ by propagation delay, the time interval between the application of an input signal and the occurrence of the corresponding output. Open-collectors have an output signal, provided by a transistor, which acts like a switch closure to ground when activated. Open-drain collectors require a pull-up resistor, use complementary metal oxide semiconductor (CMOS) technology, and represent a counterpart to transistor-transistor logic (TTL). Logic multiplexers with complementary outputs have both a true output and a complement. Output enabled (OE) devices have an enable pin for the output.
Logic Family and Form Factor
Logic multiplexers vary in terms of logic family and IC package type. Common logic families include standard, fast, high-speed and advanced CMOS; emitter coupled logic (ECL); TTL and Fairchild advanced Schottky TTL (FAST); gunning technology; and crossbar switch technology (CBT). Common package types 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, grid array package types include plastic-ball grid array (PBGA), tape-ball grid array (TBGA), and fine-pitch land grid array (FLGA). Some logic multiplexers are radiation-tolerant. Others provide protection against electrostatic discharge (ESD).
Logic multiplexers may be manufactured, tested, and used according to various standards. A selection of these standards is listed below.
MIL-M-38510/173 (Positive logic CMOS multiplexers)
SMD 5962-77052 (Positive logic CMOS 8-channel multiplexers)