Lets suppose:

I connected the output of 2 different OR logic gates:

out 1: A+B out 2: C+D

with a wire leading out such that these 2 outputs are connected.

what would these signals form to make?

If I was to somehow 'read' these what would it be?

  • 1
    \$\begingroup\$ you cannot connect gates output like that ... \$\endgroup\$ – francois P Nov 13 '18 at 21:49
  • \$\begingroup\$ but If I was to, What would happen? \$\endgroup\$ – fred Nov 13 '18 at 21:54
  • \$\begingroup\$ if you're lucky random results else IC will heat or even die in worst case. \$\endgroup\$ – francois P Nov 13 '18 at 22:06

For typical gates, if one wants to put out a high voltage and the other wants to put out a low voltage, they fight each other. Power supply current is high, the chips get hot, and the voltage on that node is at some indeterminate value between logic low and logic high.

That's what will happen in your scenario any time A+B does not agree with C+D.

  • \$\begingroup\$ would it be possible to ''find'' a Boolean expression for this combined output? \$\endgroup\$ – fred Nov 13 '18 at 21:50
  • \$\begingroup\$ You are missing the point. There is no valid "Boolean expression" because you are attempting to do a wired "OR" with chips that cannot handle it. You could do it using open collector for TTL or open drain for MOS. The expression would be what Transistor gave you in his answer. \$\endgroup\$ – StainlessSteelRat Nov 22 '18 at 23:00

From the comments:

would it be possible to ''find'' a Boolean expression for this combined output?

$$ (A+B) \cdot (\overline {C + D}) + (\overline {A+B}) \cdot (C+D) = smoke$$

No. If you short circuit two logic chip outputs and one is pulling high and the other low then the resultant voltage will be somewhere in the middle and will be an undefined logic level.


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