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TTL floating input ?

  1. Mar 4, 2008 #1
    Why floating input is considered as HIGH in TTL based logic ckts ?
  2. jcsd
  3. Mar 5, 2008 #2
    Long long ago, a TTL input (for a simple example the 7400) was the emitter of an NPN transistor.

    The base of said transistor was pulled up to Vcc (typically 5V in those days) by a resistor inside the chip.

    Grounding the input connected the emitter to 0V and turned the transistor on.

    Leaving the input floating or connecting it to 5V (usually via a resistor) turned the transistor off.

    You can find the schematic of a TTL gate in old datasheets.

    Since the input was relatively low resistance, there were none of the problems of CMOS inputs where allowing the input to float is a definite no no...
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