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An input 1 is defined from 2V to 5V and an input 0 from 0V to 0,8V while an output 1 is defined from 2,4V to 5V and output 0 from 0V to 0,4V.

Why are output logic levels defined to be more tolerant? Why not the same?

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  • \$\begingroup\$ The title seems to imply that inputs are more tolerant than the output. But then in the question, it's implied that the output is more tolerant. ?!? \$\endgroup\$ – Bort Nov 21 '17 at 21:17
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Having a closer tolerance on the output levels than on the input levels ensures that an input will correctly recognize the output of another circuit as being the intended level.

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NOTHING is EVER the same!

If the levels were the same for both, the slightest change in tolerance would cause some levels to be incorrectly interpreted.

The point is the outputs are defined to always be within the range of the inputs to guarantee any receiver (of a compatible type) will understand whatever level is being sent by the sender.

Having that tolerance band also provides you with some built in noise immunity, up to 0.4V, on the signal.

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Why are output logic levels defined to be more tolerant? Why not the same?

Rearranging the data from your question:

      Output Low     Input Low       Output High     Input High
Min                                  2,4 V           2,0 V
Max   0,4 V          0,8 V

enter image description here

Figure 1. The problem is "fan-out". Source: Electronics4U.

The point is that an output has to be able to drive multiple inputs reliably.

  • Each logic family has a maximum "fan-out" which is determined by the worse of the output high fan-out and output low fan-out. (An output may be able to pull 16 inputs low but only able to drive 11 inputs high. The fan-out specification would be 11 in this case.)
  • In Figure 1a we can see that the output (on the left) is supplying current to each of the three inputs. The effect of this is to cause the voltage on the output to droop a little. The inputs must work at the lowest value and a safety margin.
  • Similarly in (b) the more inputs we try to pull low the more the output voltage will rise. Again we need some safety margin to ensure that the transistors are definitely switched.
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  • \$\begingroup\$ Although that is a real effect and definitely worth a mention, I am not sure it answers the question since the guaranteed output voltages are usually defined at min to max load. \$\endgroup\$ – Trevor_G Nov 21 '17 at 20:30

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