Rails Are Levels - Electronic Circuits IV: Digital Logic Signals and Switch Networks Step by Step

Digital LOW and HIGH are modeled as exact voltage levels, not as new physical units. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

The low rail is a voltage

The LOW rail is 0 volts. The bit name is not a separate unit.

V_{\text{LOW}} = 0\ \text{V}

The high rail is another voltage

The HIGH rail is 5 volts. Digital state names are shorthand for voltage ranges.

V_{\text{HIGH}} = 5\ \text{V}