(Note: Problem solutions are available at

1. How are timer and counter memory similar?

2. What types of memory cannot be changed?

3. Develop Ladder Logic for a car door/seat belt safety system. When the car door is open, or the seatbelt is not done up, a buzzer will sound for 5 seconds if the key has been switched on. A cabin light will be switched on when the door is open and stay on for 10 seconds after it is closed, unless a key has started the ignition power.

4. Write ladder logic for the following problem description. When button A is pressed a value of 1001 will be stored in X. When button B is pressed a value of -345 will be stored in Y, when it is not pressed a value of 99 will be stored in Y. When button C is pressed X and Y will be added, and the result will be stored in Z.

5. Using the status memory locations, write a program that will flash a light for the first 15 seconds after it has been turned on. The light should flash once a second.

6. How many words are required for timer and counter memory?

7. A machine is being designed for a foreign parts supplier. As part of the contractual agreement the logic will run until February 26, 2008. However, after that date the machine will enable a ‘contract_expired’ value and no longer run. Write the ladder logic.