# Restrictions in input combinations of circuit?

#### Gonzalo Armbrust

Joined Jun 28, 2015
16
I have a problem in which I am asked to " find any restrictions on the allowable input combinations on A and B"

I have designed a next state truth table for the problem in the attachements, however i'm confused as to what exactly is a restriction. I noticed that for when A = 0, B=0, it oscillates between 0 and 1, and 1 and 0 when the Q current state variable is changed and the P (output) is changed. There is a similar case for when A = 1, and B=1. If i'm understanding this correctly then, would the restrictions be that A cannot be equal to 1 and B cannot be equal to zero when Q is equal to 1...? And also A and B cannot be equal to 1 when Q is equal to 0..?

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#### GopherT

Joined Nov 23, 2012
8,009
I have a problem in which I am asked to " find any restrictions on the allowable input combinations on A and B"

I have designed a next state truth table for the problem in the attachements, however i'm confused as to what exactly is a restriction. I noticed that for when A = 0, B=0, it oscillates between 0 and 1, and 1 and 0 when the Q current state variable is changed and the P (output) is changed. There is a similar case for when A = 1, and B=1. If i'm understanding this correctly then, would the restrictions be that A cannot be equal to 1 and B cannot be equal to zero when Q is equal to 1...? And also A and B cannot be equal to 1 when Q is equal to 0..?
The lighting of the afternoon sun coming into your window has not been fully captured by your camera. I suggest something with higher resolution. I can't stand looking at anything with such low resolution when I think about the beauty that was missed in your attempt to capture this moment with a measly 4.9Mb file.

On the other hand, if you are just trying to ask for help, please use a low resolution image next time.

#### Gonzalo Armbrust

Joined Jun 28, 2015
16
The lighting of the afternoon sun coming into your window has not been fully captured by your camera. I suggest something with higher resolution. I can't stand looking at anything with such low resolution when I think about the beauty that was missed in your attempt to capture this moment with a measly 4.9Mb file.

On the other hand, if you are just trying to ask for help, please use a low resolution image next time.
Here's the table I made, i'll just type it out:

present state / Next state
A | B | Q | Q | P (output)
0 0 0 0 1
0 0 1 0 1
0 1 0 0 1
0 1 1 1 0
1 0 0 0 1
1 0 1 0 1
1 1 0 0 1
1 1 1 1 0

Assuming that the table is correct, what would the restrictions be?
The circuit is just an S/R latch, with A and B fed into a NAND gate as the S input, and the value of B is the R input. I can post it if it's needed. P= Q'

#### StayatHomeElectronics

Joined Sep 25, 2008
1,073
Do you know of any restrictions to the use of an S/R latch?

#### MikeML

Joined Oct 2, 2009
5,444
What does the latch do if both Set and Reset are asserted?