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In logic circuit diagrams, I've seen various conventions for naming inputs and outputs of logic gates and combinatorial circuits. However, stateful elements like latches and flip-flops often have their "state" called Q. I suspect there is a connection with abstract Finite-State Machines from theoretical computer science, where "state" is often noted Q as well (so I asked them too :-)

But why have people picked this particular letter ?

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  • \$\begingroup\$ I've seen Q (and q) being used as state designation in general state-machine part of computer scienece too, so I guess it probably comes from that area. Still, that doesn't answer the question how Q became the designator for state in the first place. \$\endgroup\$ – AndrejaKo Dec 18 '12 at 13:33
  • \$\begingroup\$ thanks, I should have mentioned that. Indeed, what motivates my question is precisely this coincidence, which I believe is not one :-) \$\endgroup\$ – Gyom Dec 18 '12 at 13:46
  • \$\begingroup\$ This is an awesome question! I'd like to know the answer too, it would be nice if someone came up with historical facts and not conjecture. I'd like to remind people that Flip Flop existed BEFORE electronics, the electronic one taking the name from pneumatic/hydraulic equivalents. Perhaps thats where that nomenclature came from? \$\endgroup\$ – placeholder Dec 18 '12 at 15:58
  • \$\begingroup\$ German for Output or Source is Quelle, so did Germans come into the picture ? \$\endgroup\$ – Les Gregory Feb 8 at 8:43
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Alan Turing used the letter q to denote states in what came to be known as Turing machines. Presumably the q stood for quanta, emphasizing a state's discrete rather than continuous nature. This happened in the 30s when quantum theory was permeating the scientific æther.

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So a quick trip through the USPTO database is revealing.

This is only a partial answer, and the answer will be much harder to find, simply from observing the differences in what is DOCUMENTED you see a lot disparity in usage. i.e the terminology is not uniformly applied.

I traced back through Integrated semiconductor solutions to discreet and even tube systems.

  • Hughes pat # 2903606 issued '59, Filed in '55 discusses a JK FF using the J,K and Q, /Q notation.

  • Computer research corp. pat # 2644887 issued '53 filed in '50 speaks to FF's, and uses A,B,C input terminology for the logic. But does NOT use Q and /Q designing counters. Column 13 lines 1 and 2 talks to a "1" and a "0" for logical states.

  • Monroe calculating machine pat #2603746 issued '52, filed '50 uses tubes and the terminology of a & b as inputs and implements 1bit adder and subtractor. Using carry etc. Column 8 lines 56 ff talks to again logic levels as "1' and "0"

There were lots of later ones in IC's etc. but these are early and implemented using discreet components. It is very clear that the terminology predates IC's.

It is also very clear that the terminology is inconsistently used across inventions.

The "1" and "0" notation almost certainly comes from earlier work, I would conjecture the work of Boole might reference that.

attached is a list of patents I looked at, if someone else wants to look at them and follow the threads into even earlier ones. I only followed one thread through this. enter image description here

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Not a definitive answer, but the first flipflops had two inputs, to Set and Reset them respectively; another early type had a single Toggle input. That conveniently allocated the letters R,S,T for input signals, so I guess the choice was between Q and U for the output!

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  • \$\begingroup\$ before RS it was called Preset (PRE) and Clear (CLR) so after P comes Q.... tada... then R and S .. sounds more logical.. in sequence.. (as you know, Preset and CLR are still used on some legacy chips) It's a good thing we standardized, now I can read Chinese and Russian schematics. ( strictly referred to as Logic Diagrams even for Analog circuits, because they are logical symbols not the true equivalent circuit) \$\endgroup\$ – Sunnyskyguy EE75 Dec 19 '12 at 15:18
  • \$\begingroup\$ R/S just mean set and reset, they were not necessarily chosen because they are next in the alphabet \$\endgroup\$ – jbord39 Aug 23 '16 at 17:31
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The letter /Q/ is used to design the set of states that the automata can be in a specific moment, so /Q/ is also used to design the "status quo" (Lat: "the state in which").

George Mealy (1955) used Q to design the "present state"

S.C. Kleene (1951) Used q1 ... qn to represent each state, but he use /q/ because /p/ was already used.

Previous works related with finite states machines, are based on brain behaviour so the theory of "status quo" is reforced.

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Maybe Q is used because it looks similar to 'O' (for Output) but it cannot be confused with digit 0 (Zero).

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I was under the impression it had to do with the q-point (quiescent point) of a transistor, and that Q started referring to all transistor based components somehow.

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Because simply if output is designated by O , and output of the flipflop is 0, then it would get confusing , that's why.

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  • \$\begingroup\$ I guess my attempt to speak logic didn't confuse you didn't work. Imagine if the Hebrews and the Muslims invented Flip Flops 1st and we all had different symbols in reverse order קשּׂ קשּׂ \$\endgroup\$ – Sunnyskyguy EE75 Dec 19 '12 at 15:11
  • \$\begingroup\$ @Richman, good thing we don't have to all learn Arabic numbers just to do math! \$\endgroup\$ – The Photon Dec 19 '12 at 18:00
  • \$\begingroup\$ hehe and Roman Numerals wont work either in Logic.... zero was not a number then. Fhex= XV \$\endgroup\$ – Sunnyskyguy EE75 Dec 19 '12 at 20:50
  • \$\begingroup\$ "...then it would get confusing" -- That's not why, but I love the reasoning! \$\endgroup\$ – DrFriedParts Feb 5 '13 at 23:09
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Q in logic circuits represents the state of an element at time = zero (current time ) .

for example if you checked out this JK latch state table you will a symbol called Q next which means the state in the next clock cycle and so on.

so if we said Q , this means t = zero or our reference for the clock Q+1 is at t =1 (next clock) .

Hope it helps.

Also another reason for it in my opinion is for SR latch (set /reset ) was given R and S ..the next letter will be Q .

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