# Three legged NOT gate? What is this symbol?

I've come across the above schematic in a datasheet for a 4x2:1 bus switch. What exactly does that triangular symbol on S mean? It looks a lot like a NOT gate, but the third leg is confusing me.

• That 3rd (bottom) leg is a copy of the input. Don't know if is buffered or not, though. Aug 10, 2019 at 20:35
• It is the S input but the symbol is trying to pass on the idea that it has the same delay as the not-output, thus preventing spikes when switching. Aug 10, 2019 at 20:36

It is a gate with an inverted and a normal output. The idea is that the two outputs switch exactly at the same time. There is hardly any delay between them.

The symbol as shown in your diagram is rather awkwardly made. More often the following symbol is used for a combined buffer + inverted like that:

You will find these used with differential line drivers.

The following diagram has an issue that the S input bypasses the inverter.

If this logic was used as depicted, the gate connected directly to the S input would switch a fraction faster to the new state then the one which uses the S-NOT from the inverter.

Most manufacturers don't bother with that sort of details. Here is a typical diagram of a 4 output de-mux: