NOR Requires Either Closed - Electronic Circuits IV: Digital Logic Signals and Switch Networks Step by Step

A NOR gate is modeled as a pull-up with parallel pull-down branches. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Both LOW inputs leave both branches open

With both inputs LOW, neither parallel pull-down branch conducts. The output is 5 volts.

\text{LOW, LOW} \Rightarrow V_{\text{out}} = 5\ \text{V}

Either closed branch pulls low

Make one input HIGH. That one branch is enough to pull the output to 0 volts.

\text{LOW, HIGH} \Rightarrow V_{\text{out}} = 0\ \text{V}