# Logic Gates?

Discussion in 'General Electronics Chat' started by infinitig35, Mar 3, 2008.

1. ### infinitig35 Thread Starter Member

Mar 3, 2008
25
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Good morning all. Great site you guys have here. I have a homework problem that i'm stuck at. Here's the question: If logic gates are used to feed the inputs of an S-R latch, they are known as__________logic. ? Can someone help.

thanks all

2. ### S_lannan Active Member

Jun 20, 2007
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sequential logic perhaps?

3. ### mentaaal Senior Member

Oct 17, 2005
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I would guess the answer is external logic. My lecturer refers to the circuitry other than the flip-flops to be external logic

4. ### infinitig35 Thread Starter Member

Mar 3, 2008
25
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So which is the correct answer guys?

May 16, 2005
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6. ### Dave Retired Moderator

Nov 17, 2003
6,960
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Go back to your Professor/Lecturer/Teacher and ask them to be more ambiguous!

I will hazard a guess at "conditional" logic. Why?

Your question asks If logic gates are used to feed the inputs of an S-R latch, they are known as__________logic.

If you look at the section of the AAC e-book referenced by thingmaker3, you will see conditional logic gates (2 AND gates) at the input to the SR-latch (which is the 2 NOR gates). These two AND gates at the input of the SR-latch are there for conditional input selection, via input E.

Dave

7. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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My guess is combinatorial logic, but I'm not sure.

8. ### infinitig35 Thread Starter Member

Mar 3, 2008
25
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Thanks guys, I'll try to research this one out.

have a good day

9. ### Dave Retired Moderator

Nov 17, 2003
6,960
146
The answer will be something you have been taught. Are any of the answers offered of topics you have covered?

As phrased, I still believe the answer is "conditional".

Dave

10. ### infinitig35 Thread Starter Member

Mar 3, 2008
25
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Well Dave, I don't recall hearing the teacher mention conditional logic, but I have short term memory so I could forgot. Anyhow, I have couple more problems that I'll post tomorrow morning hoping you or anyone here can help me out. thanks all

11. ### Dave Retired Moderator

Nov 17, 2003
6,960
146
Conditional is probably the wrong word. The 2 AND gates have a conditional input - the enable input E - which dictates the function of the SR latch. The question is so ambiguous you probably should drop the answers into a hat and draw one at random! Let your teacher know that as well!

Dave

12. ### Secret Mover New Member

Feb 25, 2008
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Dave.. I thought your right coz its a condition.
As i recall that when there is a feedback it is a condition.

13. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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Yes, it should be "combinatorial".

14. ### Dave Retired Moderator

Nov 17, 2003
6,960
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Yes "combinatorial" was the word I was trying to think of before, not conditional

Essentially it is pre-logic (conditioning-logic) that clarifies the validity of the input variables via the enable function.

Dave

EDIT: Just seen cumesoftware's answer from yesterday stating "combinatorial".

15. ### Davidpostlethwaite Member

Mar 2, 2008
12
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the answer is 'control logic'. The logic gates outside the S-R flip flop are used to control the S-R its self. That's what we called it at uni anyway!

16. ### Dave Retired Moderator

Nov 17, 2003
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Isn't "control logic" a software construct?

Dave

17. ### infinitig35 Thread Starter Member

Mar 3, 2008
25
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Hey guys, I was at class last night and this question was brought up to the teacher. He gave us a hint the answer has the word "pulse" in it. He said the second word contain an "s" in the first letter.

18. ### beenthere Retired Moderator

Apr 20, 2004
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Using a gate to control inputs conditionally is called steering logic.

19. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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If the S & R inputs are fed into (AND) gates with a clock input controlling the gating of each, then it is a synchronous system and not combinatorial. The answer is probably "synchronous pulse" or some version thereof, such as "pulse synchronizing".

This can be used in large digital systems where all latches are to switch states effectively in unison (although today it's not the best way).

The instructor's ambiguity in the question leaves a lot to be desired. I guess that's why you have to attend class.

20. ### Dave Retired Moderator

Nov 17, 2003
6,960
146
I have come across asynchronous pulse logic in papers, but not synchronous pulse logic (thats not to say it doesn't exist). From the level of the question, I struggle to see that the tutor would have covered this. Sounds like banging a few words into Google and seeing what comes out.

For all intents purposes, the description you reference from the OP sounds like a gated SR latch: http://sub.allaboutcircuits.com/images/04178.png

Dave