Suppose a TTL output in high impedance is connected to a LED, which is also connected to a resistor, which is then connected to ground. No circuit interferes between TTL output and LED, between LED and resistor, and between resistor and ground.

In this case, what would be logic value of TTL output?

  • \$\begingroup\$ This question is similar to this: "If a tree falls in a forest and no one is around to hear it, does it make a sound?". If you explain exactly what you mean by sound, it is a very simple question. - If you explain exactly what you mean by high impedance, then this is also a very simple question. Does high impedance mean that internal pull-up resistors are used? Does high impedance mean that the output is floating? - A very simple question indeed. If we properly define things. I know what it is. But do you know what it is? \$\endgroup\$ – Harry Svensson Apr 21 '18 at 2:19
  • \$\begingroup\$ THe logic value is called _tristate "x" \$\endgroup\$ – Sunnyskyguy EE75 Apr 21 '18 at 3:44
  • \$\begingroup\$ what would be logic value of TTL output ... there is none (also known as hi-Z, tri-stated, or floating) ... the output is effectively disconnected .... if you connect the output to an input, then the input would actually be floating \$\endgroup\$ – jsotola Apr 21 '18 at 3:45

Using a very high impedance voltmeter, you'd measure a forward-LED-diode-drop as appropriate for 1nanoampere of leakage from the Hi-Z logic output. Or at high temperatures, a forward-LED-diode-drop suited for 1 microampere of leakage.

The voltage would be in the forbidden zone, between 0.8v and 2.5v.


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