# Implementing three-state logic

Can we build a three-state logic gate using only transistors (or basic logic gates)? If so, how is it built?

I'm trying to build a model for an 8-bit computer in LogiSim, and I'm trying to find out if it is possible to build everything using transistors, voltage sources, and a clock.

I know that a multiplexer can be used instead, and that it can be implemented by logic gates, but I prefer to use three-state logic gates.

• When you say 3 logic states, are you implying 3 voltage levels? Commented Oct 25, 2022 at 21:13
• Tri nary or tristate (high, low, hi-z)? Commented Oct 25, 2022 at 21:14
• A tri-state output can be made discretely from one NOR, NOT, and NAND gate, and two logic-level MOSFETs as seen here by Terry Sturtevant. Commented Oct 25, 2022 at 21:56
• High, low, and high-Z is not the same as ternary logic. Commented Oct 25, 2022 at 23:58
• Does this answer your question? Non-Inverting Tri-State buffer with transistors only possible?
– CL.
Commented Oct 26, 2022 at 8:15

Yes. The logic gates with three state outputs are also made of transistors.

Logisim has transistors, but it also has buffers and inverters with three state output.

A three state output is like a standard push-pull output, but both the high side and low side output transistors are separately controlled to let them either output a signal in push/pull mode or keep both of them off.

• Thanks. Do you have some source showing how it is built? I know that I can just use the tools from Logisim, but I'm interested in building them myself fom scratch. Commented Oct 25, 2022 at 21:21
• @RdBasha Wouldn't you just go to the Texas Instruments databook on the 7400 series devices and examine their schematics for "gates with 3-state outputs?" It's there, I believe. I forget what chapter, but it is roughly in the middle of the book.
– jonk
Commented Oct 25, 2022 at 22:16
• @jonk - You're assuming that the hi-Z state is a 3rd logic level. I'm not sure that's what OP was asking about, and hence my first comment. Note that Intel way back in the 70's actually implemented ternary logic (3 voltage levels) in one of the PROMs ,IIRC. Commented Oct 25, 2022 at 23:57
• @RdBasha Do you have some source showing how it is built? ... you still have not clarified what you are actually asking about Commented Oct 26, 2022 at 1:08
• @SteveSh I did make that assumption. And it could be that one of the trinary (balanced or otherwise) concepts was implied. That said, 'Rd' did happen to respond to the answer here discussing 3-state outputs and didn't disagree. So that may be a small bit of evidence to consider in addition to the question, itself.
– jonk
Commented Oct 26, 2022 at 2:59