 # List of PLC Practice Problems and Solutions

In this forum discussion, you can find the list of PLC practice problems and their solutions.

## PLC Practice Problems

1. Give an example of where a PLC could be used.

2. Why would relays be used in place of PLCs?

3. Give a concise description of a PLC.

4. List the advantages of a PLC over relays.

5. A PLC can effectively replace a number of components. Give examples and discuss some good and bad applications of PLCs.

6. Explain why ladder logic outputs are coils?

7. In the figure below, will the power for the output on the first rung normally be on or off? Would the output on the second rung normally be on or off?

1. Write the mnemonic program for the Ladder Logic below.

## PLC Practice Solutions

1. To control a conveyor system

2. For simple designs

3. A PLC is a computer based controller that uses inputs to monitor a process, and uses outputs to control a process using a program.

4. Less expensive for complex processes, debugging tools, reliable, flexible, easy to expand, etc.

5. A PLC could replace a few relays. In this case the relays might be easier to install and less expensive. To control a more complex system the controller might need timing, counting and other mathematical calculations. In this case a PLC would be a better choice.

6. The ladder logic outputs were modelled on relay logic diagrams. The output in a relay ladder diagram is a relay coil that switches a set of output contacts.

7. off, on

8. Generic: LD A, LD B, OR, ST Y, END; Allen Bradley: SOR, BST, XIO A, NXB, XIO B, BND, OTE Y, EOR, END

## PLC Assignment Problems

1. Explain the trade-offs between relays and PLCs for control applications.

2. Develop a simple ladder logic program that will turn on an output X if inputs A and B, or input C is on.