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I am struggling to understand how the SR latch works.

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When an input has designation 0, this means "low signal" correct? Is this completely different than "no signal"? Doesn't a logic gate need some kind of signal in both input wires in order to produce an output signal?

If so I don't understand how an SR latch can ever product any kind of output. Even if we sent signal in through S and R at the exact same time, neither gate can produce an output until the other input on each gate receives a signal, but both gates rely on each other for those inputs, so...?

At the same time it can't be "no signal" either because this would imply the NOR gates could send a signal even when no power is being sent through.

Where is my understanding wrong?

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2 Answers 2

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When an input has designation 0, this means "low signal" correct?

Uh, usually. Sometimes people will work with "negative true" or "active low" logic, which doesn't change the parts, it only changes the meaning of the voltages.

Is this completely different than "no signal"?

Yes and no. When you're talking boolean logic there's only two possible states. When you're talking electronics, the "gates" act like high-gain amplifiers.

Doesn't a logic gate need some kind of signal in both input wires in order to produce an output signal?

Uh -- beep, does not compute. See above. There's always a "signal".

If so I don't understand how an SR latch can ever product any kind of output. Even if we sent signal in through S and R at the exact same time, neither one can produce an output until the other input receives a signal, but they both rely on each other for this.

Look at the logic elements as if they're circuit elements. When you turn on the power, the latch is in some unknown state, or it may even be in a real live indeterminate state, with both outputs stuck in between low and high. Now think of those two NOR gates as amplifiers with negative gains of amplitude more than one. If one of those outputs is just a bit above center, it'll cause the other output to drive down, which will drive the one output even higher, and the latch will quickly settle into one state or another.

(A similar thing happens if you tie the S and R inputs together and toggle them simultaneously -- with both inputs high, the outputs are held low, but it's like balancing a penny on a needlepoint. As soon as the inputs go low the outputs will fight briefly, then the latch will go to one state or another).

At the same time it can't be "no signal" either because this would imply the NOR gates could send a signal even when no power is being sent through.

There is no question mark here, but I think you're unclear on the concept of power vs. the concept of signal. In the real world, the gates would be powered; in the theoretical world, the gates just magically act the way they do, without requiring power.

Where is my understanding wrong?

I'm not sure. But I hope that I've clarified things.

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  • \$\begingroup\$ "When you turn on the power, the latch is in some unknown state, or it may even be in a real live indeterminate state, with both outputs stuck in between low and high" Why? What does this mean exactly? How can it be stuck in some partial state? \$\endgroup\$
    – user204396
    Commented Nov 15, 2018 at 4:24
  • \$\begingroup\$ I did not say it would be stuck in some partial state. I said that it would be in some indeterminate state. This means that it is not in a state where one output is high and the other is low. Generally the latch gets out of such states fairly quickly, which is why I compared that condition to being balanced on a needle-point. \$\endgroup\$
    – TimWescott
    Commented Nov 15, 2018 at 4:36
  • \$\begingroup\$ I still don't understand how an output signal is generated in the first place or how the circuit could be in an indeterminate state \$\endgroup\$
    – user204396
    Commented Nov 15, 2018 at 4:38
  • \$\begingroup\$ I suggest that you write out the schematic on paper, with a pencil. Then assign ones to both inputs, and zeros to both outputs, and ask yourself "what will happen?" Then write a little 's' next to one of the gates and say "that's the slow gate", and repeat the exercise. \$\endgroup\$
    – TimWescott
    Commented Nov 15, 2018 at 4:40
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When an input has designation 0, this means "low signal" correct?

Yes.

Is this completely different than "no signal"?

There is no such thing as "no signal".

Doesn't a logic gate need some kind of signal in both input wires in order to produce an output signal?

There's always a signal so this question is moot.

If so I don't understand how an SR latch can ever product any kind of output. Even if we sent signal in through S and R at the exact same time, neither gate can produce an output until the other input on each gate receives a signal, but both gates rely on each other for those inputs, so...?

There's always a signal so this question is moot.

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  • \$\begingroup\$ How can a gate produce a high signal if both inputs have no power (also "low signal" in this case)? \$\endgroup\$
    – user204396
    Commented Nov 15, 2018 at 4:26
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    \$\begingroup\$ "No power" is an entirely different thing than "low signal" for the purposes of this discussion. It means something if you're talking about a radio receiver, but not if you're talking about logic gates. Sweep it out of the "thinking about logic gates" room in your brain, and put all the pieces into the "thinking about radio" room. \$\endgroup\$
    – TimWescott
    Commented Nov 15, 2018 at 4:38
  • \$\begingroup\$ Real logic gates require a power supply, in addition to the logic inputs. You may think of the logic inputs as controlling the output stage. The output stage of the gate uses the power supply to generate the output logic signal. \$\endgroup\$
    – Peter Bennett
    Commented Nov 15, 2018 at 4:42
  • \$\begingroup\$ @user204396 Because its power supply has power? How can your computer turn on when you don't type on the keyboard? \$\endgroup\$
    – user20574
    Commented Nov 15, 2018 at 10:40
  • \$\begingroup\$ @immibis Because I didn't know if "low signal" was different from "no signal", as in a system where no current flowing through somehow meant there was no power and therefore no activity, and then everything receiving low power was "low signal" etc. As in everything being connected but sending a "low signal" representing 0 when "not active" \$\endgroup\$
    – user204396
    Commented Nov 15, 2018 at 10:48

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