## 1.5 PRACTICE PROBLEMS

1 Develop the ladder logic that will turn on an output light (201), 15 seconds after switch A (001) has been turned on.

2. Develop the ladder logic that will turn on a light (201), after switch A (001) has been closed 10 times. Push button B (002) will reset the counters.

3. Develop a program that will latch on an output B (201), 20 seconds after input A (001) has been turned on. The timer will continue to cycle up to 20 seconds, and reset itself, until input A has been turned off. After the third time the timer has timed to 20 seconds, the output B will be unlatched.

4. 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.

5. A motor will be connected to a PLC and controlled by two switches. The GO switch will start the motor, and the STOP switch will stop it. If the motor is going, and the GO switch is thrown, this will also stop the motor. If the STOP switch was used to stop the motor, the GO switch must be thrown twice to start the motor. When the motor is running, a light should be turned on (a small lamp will be provided).

8. For the ladder diagram below, what does the program do? (Note: the keep is equivalent to an S-R flip-flop, in other words S will latch the value on, and R will unlatch the value). Note that 200 and 210 are lights and 202 is a motor. 100, 102 and 104 are input push buttons.

(ans. this is a motor controller. 100 is a start button, and 102 is a stop button. Light 200 will indicate when the motor is on. Light 210 will indicate when either start or stop is pushed, or the emergency button is not pushed. The motor 202 will run if the start button has been pushed and the normally closed stop switch is not pushed).

9. Counters are used to count events or occurrences of inputs when a transition from OFF-to-ON occurs, which implies that they are ________________________ triggered.

(ans. positive edge)

10. The address for a memory location that indicates when a PLC has just been turned on is ___________________.

(ans. 868 for SLC-150)

11. We are developing a safety system (using a PLC-5) for a large industrial press. The press is activated by turning on the compressor power relay (R, connected to O013/05). After R has been on for 30 seconds the press can be activated to move (P connected to O013/06). The delay is needed for pressure to build up. After the press has been activated (with P) the system must be shut down (R and P off), and then the cycle may begin again. For safety, there is a sensor that detects when a worker is inside the press (S, connected to I011/02), which must be off before the press can be activated. There is also a button that must be pushed 5 times (B, connected to I011/01) before the press cycle can begin. If at any time the worker enters the press (and S becomes active) the press will be shut down (P and R turned off). Develop the state transition diagram, and then ladder logic for the states. State all assumptions, and show all work.

12. Given the following timing diagram, draw the done bits for all four fundamental timer types. Assume all start with an accumulated value of zero, and have a preset of 1.5 seconds.

13. Write ladder logic that will give the following timing diagram for ‘B’ after input ‘A’ is pushed. After ‘A’ is pushed any changes in the state of ‘A’ will be ignored.