Question

another method of multiplexing involves the use of tri-state devices.   First of all, how are multiplexers used and second, what are tri-state devices and why do we really care about tri-state device (yes we most emphatically DO CARE.. they are very important).

Answer 1

What is Multiplexer

Multiplexing is the generic term used to describe the operation of sending one or more analogue or digital signals over a common transmission line at different times or speeds and as such, the device we use to do just that is called a Multiplexer.

Refer this fig to get an idea:

Basically, Multiplexing can also be defined as a technique that allows simultaneous transmission of multiple signals across a single data link.The input selected for connection to the output is controlled by a set of SELECT inputs.

Usage of Multiplexers:

Multiplexing techniques can be categorized into the following three types:

Frequency-division multiplexing (FDM): It is most popular and is used extensively in radio and TV transmission. Here the frequency spectrum is divided into several logical channels, giving each user exclusive possession of a particular frequency band.

Time-division Multiplexing (TDM): It is also called synchronous TDM, which is commonly used for multiplexing digitized voice stream. The users take turns using the entire channel for short burst of time.

Statistical TDM: This is also called asynchronous TDM, which simply improves on the efficiency of synchronous TDM.

What is tri-state?

A tri-state buffer is a device that allows you to control when an output signal makes it to the bus. When the tri-state buffer's control bit is active, the input of the device makes it to the output. ... When it's not active, the output of the device is Z, which is high-impedance or, equivalently, nothing.

These are the examples of tri-states devices :

1)Part no. 74LS125A Quad 3-state noninverting buffer 74LS125A

2)part no.74LS366A Hex 3-state inverting buffer with common enables 74LS366A

We really care about because of these reasons,

Tristate logic elements provide a solution to the problems of speed and power in bus organized digital systems. Tristate gates are essentially gates with output stages that assume three states. Two of these three are normal low impedance High and Low states. The third one is a high-impedance (Hi-z) state. When the device is in Hi-z state both the transistors in the output totem-pole circuit are in off conditions. When the output of such a gate in Hi-z state is tied to the output of a gate that is in Lo-z state, the High-z state gate does not influence (in any significant manner) the output circuit of a Lo-z state gate. This enables us to tie the outputs of many tristate devices, and share a common (bus) signal line. These units have the speed of the regular devices, higher line-drive capability and higher noise immunity. By eliminating the pull-up resistors these tristate gates cut bus delays to a few nanoseconds.