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3.5 TRANSMISSION GATES AND TRI-STATE DRIVERS 85

EN HV LV
J^ Passes LV well
X _ i — i x but not HV in ^ i t x =X X _ • « _ X
1 j
NMOS ° transfer (ON) mode o i i o
ON OFF
LV HV
Passes HV well
Du n
t °t LV in x „ „ y _ V Y
transfer (ON) mode ' °~ ' ]

EN _ M HV LV
i EN
I I Passes both LV
— £gj — and HV well in X ; — PI — X 0 =Xj X. — I I — X 0
CMOS — transfer (ON) mode I
EN EN LV HV
ON OFF
(a) (b) (c)

FIGURE 3.7
Transmission gate circuit symbols and their idealized equivalent circuits, (a) Simplified circuit
symbols for NMOS, PMOS, and CMOS transmission gates, (b) ON (transfer) mode equivalent circuit.
(c) OFF (disconnect) mode equivalent circuit.

data bus so that the devices will not interfere with each other. By this means, tri-state drivers
permit multiple signal sources to share a single line if only one of the signals is active at any
given time. These drivers also serve as a controlled enable on the output of some devices.
Note that the term "tri-state" is a trademark of National Semiconductor Corporation. Thus,
the use of the term "tri-state" in this text acknowledges NSC's right of trademark. The terms
tri-state and three-state are often used interchangeably.
Shown in Fig. 3.8 are four types of CMOS tri-state drivers constructed from the equivalent
of two or three inverters. They differ in the activation levels of the control input, C, and the
output, X 0, indicated by the logic circuit symbols. The choices are inverting or noninverting
tri-state drivers with active high or active low control inputs, as provided in Figs. 3.8a-d. The
buffering (driving) strength is the same for all tri-state drivers, however. This is so because
during the transfer stage the outputs X 0 are connected to the supply +VDD depending on
the character of the driver and the X/ voltage level. For example, in the case of the inverting
tri-state driver of Fig. 3.8c, a control C = HV connects the output X 0 to +VDD if the input
is X f = LV or connects X 0 to ground if X,• = HV. Thus, in the transfer mode, the transistors
of a tri-state driver serve as transmission gates, thereby permitting an input signal to be
enhanced (or refreshed); hence the meaning of the term driver. Of course, in the disconnect
mode the tri-state driver produces a very large impedance (Hi-Z) between its input and
output, virtually disconnecting the input from the output.
Note that the conjugate logic circuit symbols are provided for each tri-state driver shown
in Fig. 3.8 and that these symbols are interchangeable — as they must be, since they are
derived from the same physical device (the tri-state driver). The idea here parallels that of
the inverter and its conjugate logic circuit symbols shown in Fig. 3.6. Symbol X appearing