Tristate Buffer Gate


Another type of special gate output is called tristate, because it has the ability to provide three
different output modes: current sinking ("low" logic level), current sourcing ("high"), and °oating
("high-Z," or high-impedance). Tristate outputs are usually found as an optional feature on buffer
gates. Such gates require an extra input terminal to control the "high-Z" mode, and this input is
usually called the enable.

Tristate buffer gate

With the enable input held "high" (1), the buffer acts like an ordinary buffer with a totem pole
output stage: it is capable of both sourcing and sinking current. However, the output terminal °oats
(goes into "high-Z" mode) if ever the enable input is grounded ("low"), regardless of the data signal's
logic level. In other words, making the enable input terminal "low" (0) effectively disconnects the
gate from whatever its output is wired to so that it can no longer have any effect.
Tristate buffers are marked in schematic diagrams by a triangle character within the gate symbol
like this:

Tristate buffer symbol

Tristate buffer symbol

Truth table

A B Output
0 0 High-Z
0 1 0
1 0 High-Z
1 1 1

Tristate buffers are also made with inverted enable inputs. Such a gate acts normal when the
enable input is "low" (0) and goes into high-Z output mode when the enable input is "high" (1):

Tristate buffer with inverted enable input

A B Output
0 0 0
0 1 High-Z
1 0 1
1 1 High-Z

One special type of gate known as the bilateral switch uses gate-controlled MOSFET transistors
acting as on/off switches to switch electrical signals, analog or digital. The "on" resistance of such
a switch is in the range of several hundred ohms, the "off" resistance being in the range of several
hundred mega-ohms.

Bilateral switches appear in schematics as SPST (Single-Pole, Single-Throw) switches inside of
rectangular boxes, with a control terminal on one of the box's long sides

See Bilateral switche

Keywords : Tristate, Buffer, Gates, MOSFET, Transistors, CMOS, SPST, Single-Pole, Single-ThrowTruth, TableCMOS-gates, Logic Gate, Symbol
Writer : delon  |
26 Nov 2006 Mon   
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