# Dgital Circuits problem:-Help me to solve Sequential counter problem

#### riya

Joined Jan 12, 2013
4
To open a security door, the code 2 7 2 0 has to be entered on a keypad. The numbers are entered
one by one. The user may press arbitrary keys (0. . . 9) on the keypad; on an erroneous input, a warning
signal W should be given (W = 0: no erroneous input; W = 1: erroneous input). On entering the
next digit, the signal W is set to 1 or 0 again, depending on the former correctness of the current input
(including the new digit). When the code 2 7 2 0 is entered completely in the right order, the lock
should be opened by setting the signal E = 1. Note that there may be digits which when entered make
the current input erroneous but which also belong to the correct code. This correct code needs to be
recognized, too. Additionally the signal W needs to be set to 1 on entering the digit which makes the
current input erronenous. After that, processing may continue, considering the past input. The door
closes and locks itself automatically after use, until a correct code unlocks it again. Design a sequential
circuit for controlling the lock:
(a) Identify the inputs and outputs of the automat. Develop a wise coding for the inputs (the digits
of the keypad) to keep the expense small. If a combinational circuit is needed for the coding, also
state it (with drawing).
(b) Draw the complete Automatengraph
(c) Give the corresponding truth table and minimize the output functions and state transitions
(d) Draw the corresponding sequential circuit of the automat as FPLA using D-Filp flops.

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#### riya

Joined Jan 12, 2013
4

#### tindel

Joined Sep 16, 2012
714
This sounds like a contract job... My rate starts at \$75 an hour.

We offer help - you do the legwork. After you start doing the work and run into problems, then ask for help.

#### zaman999

Joined Jan 29, 2011
26
Post this in the homework help forum.

You can't expect someone to solve the entire problem for you. Ask specifically where you're getting stuck or where you have a difficulty in understanding. Then you might get some help.

#### WBahn

Joined Mar 31, 2012
25,556
This is a forum in which people from around the world participate as they have time. To get impatient and post a follow-up because no one has answered your first post after just four minutes is to show that you don't understand how forums work (which is fine -- everyone here had to learn at some point). Don't be surprised if your thread sits for a couple days without getting a response -- my first post sat for considerably longer than that!

We are willing to help you with your homework, hence why the forum is called Homework Help. It is not called the Homework Done For You foum. You need to post your attempt(s) at solving your problem (since it is YOUR problem). We can then see how you are approaching the problem and where you are going astray and can then help get you back on track.

If you haven't already, perhaps you might start with a state diagram that goes from an initial condition (i.e., presently 0 correct entries) to the final condition (i.e., presently 4 correct entries) and show the transitions that would take you from the first to the last in order. Then, from each state, draw arrows for each of the possible entries and which state they would go to.

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#### riya

Joined Jan 12, 2013
4
I am really sorry.. I am new in this forum. I am student and I solved this problem bt i could not attach my solution.

I am really sorry what I have done.

#### WBahn

Joined Mar 31, 2012
25,556
Nothing to be forgiven for -- you've just learned something about how forums work, that's all. Glad to hear you got the problem solved on your own. That's really the best way. If you would like your solution critiqued to see where you might have missed something or where you might have considered an alternate approach, just post your solution (or whatever part is relevant) and try to form some reasonably specific questions. There's a lot of good help to be had here, but there is a learning curve involved in figuring out the most effective way to tap into it.

#### riya

Joined Jan 12, 2013
4
Thanks to support and guide me.

I am sending my solutions as an attachment .I hope it will be uploaded.

again I am really sorry and thanks for guiding me.

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#### WBahn

Joined Mar 31, 2012
25,556
You've got a pretty good start, but I think you have a number of errors in your state diagram. For instance, your state 011 appears to be your accepting state (i.e., the entire code has been entered successfully), yet you show many different ways to get there, including directly from the Start state by entering 0. You also show that the person will remain in State 010 (meaning that 2-7 has been entered and that the next correct entry is a 2) as long as the person keep hitting 7. So this will accept 2-7-7-7-7-2-0 as a valid combination. Is that what you want? It will also accept 2-7-0 as a valid combination.