My Solution for CLD Sample Exam (Traffic Light)

Question

Hi there folks,
I was just preparing for my CLD exam and was going through the sample exam provided on ni.com.
I wanted to get any feedback for my solution. I looked at the solution and it seemed way to complex
to achieve a simple task! Then again, I am still learning.
ThanX in advance.
Attachments:
Traffic Light1.zip ‏81 KB
Traffic Light2.zip ‏81 KB

Answer 1

About queues vs shift registers.  Queues allow you the ability to insert elements into either end of the queue.  You can also write code to insert several elements at one time.  With shift registers, only one new element can be carried over to the next loop at one time.
Lets say you have a state machine that always executes in the same order, no matter what the outcome, like State1 then State 2 then State 3.  A shift register works fine for this simple example.  However with queues, you can use a loop to insert the 3 states before the main state machine loop, then dequeue in the state machine.  This is really a matter of preference.
Next example.  Lets say you had a situation where you want to run 3 states, but if the second one fails, you want to run 2 other states before going to the third one.  You could use a shift register, and you would have to look at each state to see which is next.  A queue would allow you to insert the 2 extra states in between states 2 and 3.  Of if you want to exit on some failure during a state.  You can use the queue to insert the exit state at the opposite end so that it is the next to be dequeued.
Another situation is if you allow the user to call certain actions that require running several states.  When the user presses button A, you enqueue states 1,2,3.  If the user presses button C you enqueue states 7,8,9.  So now you have a state machine in which states are defined by some user action.  This would be more difficult to do with a shift register. 
There are many other situations where a queued state machine is better than just a simple shift register.  For the simplest of situations, I might use a shift register only.  But I like to make it a habit to always use queues because of their versatility, and for the fact that I can go back and upgrade a simple state machine to a more complex one easily if it is done with queues.  Be sure to use shift registers on the queue error in and error out so that errors can be propogated from one state to the next.  On each state, check the error status.  If one occures, enqueue a state at the other end to jump to your error handling state (or exit).
- tbob
Inventor of the WORM Global