Question on State Table Outputs

Devices with memory produce outputs at time T+1 based on the outputs and inputs at time T. The outputs are locally stored inputs.

There are two major state machine types : Moore and Mealy.

Moore : output is only dependent on the current state

Mealy : output is a combinatorial function of current state and current input(s)

So, for your example, Output column should be for time (T), not (T+1). You may take a look at Randy H Katz Contemporary Logic Design book from '93, available online at

Have fun!

Francois Choquette

Best Match for a high school student's needs.

Hi. I'm taking a course on introductory digital logic, and there is a question where I have a disagreement with the instructor, and I think he may be mistaken on a certain point. We are using the text Digital Design by Mano, and while the book is generally very good, I think even the text is not fully clear on this.

Suppose you have a State Table for a simple circuit with, say, two flip flops (A and B), input X, and output Y. So the headings for the table are:

Present State (time T) Input (T) Next State (T+1) Output (T+1) A_______B__________X_______ A____ B_____________ Y

Now, the question is right there in the table heading, because the instructor says that the output column, Y, should be for time T -- that is, it is based on the states of A and B at time T. To me that makes no sense -- it seems the whole point is to start with A and B at time T, put in your input X, and see what A, B, and Y are at time T+1. So, that's basically the question -- is the output column for time T, or for time T+1?

Thanks in advance for all replies.

Steve O.

"Spying On The College Of Your Choice" -- How to pick the college that is the Best Match for a high school student's needs.

If Y is a combinatorial logic function of A and B _only_, then it doesn't matter whether you tabulate Y(T) or Y(T+1) as long as you say which it is. You've got A(T), B(T), A(T+1) and B(T+1) all tabulated on the same line.

Y(T) = f( A(T), B(T) ) Y(T+1) = f( A(T+1), B(T+1) )

i.e. Y(n) = f (A(n), B(n))

I thought it was implied that since we have flip-flops, the logic is sequential and not combinatorial. But if I make that explicit -- this is sequential logic -- then how does the answer to my question come out?

"Spying On The College Of Your Choice" -- How to pick the college that is the Best Match for a high school student's needs.

Steven O. wrote

State machines contain combinatorial logic and flip-flops. The sum of the parts is sequential logic.