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Digital Electronics
Figure 5.46 CMOS inverters with shorted outputs.
Y=A
_
A
Output
protection
diode
+V DD
Figure 5.47 CMOS inverter with an open drain output.
5.5.1.11 CMOS with Tristate Outputs
Like tristate TTL, CMOS devices are also available with tristate outputs. The operation of tristate
CMOS devices is similar to that of tristate TTL. That is, when the device is enabled it performs its
intended logic function, and when it is disabled its output goes to a high-impedance state. In the highimpedance state, both N-channel and P-channel MOSFETs are driven to an OFF-state. Figure 5.48
shows the internal schematic of a tristate buffer with active LOW ENABLE input. The circuit shown
is that of one of the buffers in CMOS hex buffer type CD4503B. The outputs of tristate CMOS devices
can be connected together in a bus arrangement, like tristate TTL devices with the same condition that
only one device is enabled at a time.
5.5.1.12 Floating or Unused Inputs
Unused inputs of CMOS devices should never be left floating or unconnected. A floating input is highly
susceptible to picking up noise and accumulating static charge. This can often lead to simultaneous