There are
instances wherein the output of a CMOS logic gate needs to be used for
driving the input of a TTL gate. Since the voltage-current
characteristics and requirements of a CMOS gate differ from those of a
TTL gate, it is good practice to use proper interfacial components
between them when connecting them to each other. Below are some
common techniques used in connecting a CMOS gate to a TTL gate.
Figure 1.
Interfacing any CMOS gate to any TTL gate using the same power supply
(5V)
When the CMOS
gate that will drive the TTL gate also uses the same 5-V supply used by
the TTL gate, the simple interfacing technique shown in Figure 1 may be
employed. Here, a pull-down resistor is just placed between the CMOS
gate output and ground.
Figure 2.
Interfacing
an Open-Collector TTL gate to any CMOS gate using different power
supplies
When the CMOS
gate
that will drive the TTL gate
has a supply voltage
that's different from the
5-V supply used by the TTL gate,
it would be
good to use an NPN transistor to translate
the CMOS output voltage level to a correct TTL input voltage level as
shown in Figure 2. Note that the transistor uses the 5-V TTL
supply for its Vcc.
Figure 3.
Interfacing
any TTL gate to any CMOS gate using
different
power supplies
As an
alternative to the technique shown in Figure 2, the technique shown in
Figure 3 may be employed to connect a CMOS gate to a TTL gate.
Instead of a transistor, a CMOS buffer (inverting or non-inverting) may
be used as long as it is supplied from the 5-V TTL supply. The example
in Figure 3 is an inverting buffer, so the input to the TTL gate is an
inverted logic of the CMOS output.
See Also:
TTL-to-CMOS Interfacing Techniques;
Logic Gates;
TTL
See
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