Are these valid logic circuits?

Question

This [Circuit A] is allowed in Digital:

This [Circuit B] is also allowed:

Until I turn off one of the inputs:

This [Circuit C] causes a different error, depending on whether the input is on or off.

When it is off - I get an error about oscillation:

When the input is on - I get an error about a short circuit:

My questions are:

1. Is [Circuit A] actually valid, i.e., would it not lead to a short circuit in real life? Or is Digital incorrectly not detecting this as error?

2. Is [Circuit B] valid (i.e. no short circuit) as long as the inputs are both on or off?

3. It's clear to me that [Circuit C] is invalid, but I wonder if Digital points out the correct consequences in the error messages. Would the circuit really oscillate when the input is off, and short when the input is on? And if so - why the difference in behavior?

Answer 1

No, none of the circuits are valid.

You can't connect two or more outputs together.

There is a problem if one output is logic '1' and the other output is logic '0'. The result is arbitrary or invalid.

(Exceptions apply. In some scenarios such a case could be defined what it is and thus valid. Like wired-and / wired-or circuits).

So you can't think something is on or off, because for many people on means something is actively happening and off means nothing is happening so it is irrelevant.

In this case, logic state '0' is as just as strong output than logic state '1', and you can't have a logic state 'half' by connecting logic '0' and logic '1' outputs together.