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I was working with IC 7400 (or LM74LS00 / NAND) (and IC 7486 - XOR) to design and verify combination circuits and logic gates.

Consider the lower end of breadboard as LOW (connected to GROUND) and upper end as HIGH (+5V).

When I connect the input to GROUND (like putting the wire on lower end on breadboard), it is considered as LOW and works as LOW; and when I connect the input to HIGH, it works as HIGH (normal behavior).

But when I remove the wire from both HIGH and GROUND, it still behaves as HIGH.

Why it happens so?

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The input stage of a TTL device acts as a reverse-biased diode. As such, any high impedance at the input stage, whether due to expansion of the depletion zone of the B-E junction or due to an entirely disconnected pin, will appear as a high input.

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Have a look at the simplified diagram of a NAND gate from wikipedia. You can see that a current can flow from Vcc trough the base and collector of the first transistor to the base of the second transistor and switch it on.

Simplified NAND gate

This example shows once again how important it is to define the levels properly and connect unused inputs to GND or Vcc. Because the inputs of regular TTL logik (not LS) are sensible to over voltage, Texas Instruments recommends to use a 1K Ohm pull-up resistors that you can use for up to 25 inputs. The inputs of unused gates, flip-flops, counters etc. should be connected to GND.

With common TTL components open inputs can be fairly reliable assumed as HIGH, but when switching to LS or even CMOS logik with it's high impedance inputs you will certainly run into problems. Capacitive coupling and electrostatic charges will change your bits in an unpredictable way.

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From TI's TTL NAND data sheet, reproduced below, it can be seen that with the inputs (A,B) either both high, or allowed to float, current will be allowed to flow into the base of the totem pole driver, eventually driving the output (Y) low.

However, when either A or B is pulled low, the current which would flow into the base of the totem pole driver is, instead, diverted to ground, forcing Y high.

enter image description here

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