Wednesday 7 March 2012

Basics Review Continues ....


Triodes:

A tri-state device is normally a digital device. It means the device is in the high state, the low state, or in a high impedance state to prevent loading.


Three-state logic:
 In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the output from the circuit. This allows multiple circuits to share the same output line or lines (such as a bus).
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits. Other typical uses are internal and external buses in microprocessors, memories, and peripherals.


Fig 1. Tri-state Devices

A tri-state buffer has two inputs: a data input x and a control input c. The control input acts like a valve. When the control input is active, the output is the input. That is, it behaves just like a normal buffer. The "valve" is open.

Table 1
X
C
Z
0
0
Z
1
0
Z
0
1
0
1
1
1



Decoders:
Discrete quantities of information are represented in digital systems by binary codes. A binary code of n bits is capable of representing up to 2" distinct elements of coded information. A decoder is a combinational circuit that converts binary information from n input lines to a maximum of 2n unique output lines. If the n-bit coded information has unused combinations, the decoder may have fewer than 2n outputs.

The decoder presented here is called n-to-m line decoders, where m 2". Their purpose is to generate the 2n (or fewer) minterms of n input variables. The name decoder is also used in conjunction with other code converters, such as a BCD-to-seven-segmented decoder.





Fig.2 A three-to-eight-line decoder





Table 2