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
