A Level Application

Now that we've seen how registers work, let's process a program like PLCs do to enhance our understanding of how the program gets scanned.

Let's consider the following application:
We are controlling lubricating oil being dispensed from a tank. This is possible by using two sensors. We put one near the bottom and one near the top, as shown in the picture below.

Here, we want the fill motor to pump lubricating oil into the tank until the high level sensor turns on. At that point we want to turn off the motor until the level falls below the low level sensor. Then we should turn on the fill motor and repeat the process.

Here we have a need for 3 I/O (i.e. Inputs/Outputs). 2 are inputs (the sensors) and 1 is an output (the fill motor). Both of our inputs will be NC (normally closed) fiber-optic level sensors. When they are NOT immersed in liquid they will be ON. When they are immersed in liquid they will be OFF.

We will give each input and output device an address. This lets the plc know where they are physically connected. The addresses are shown in the following tables:

Inputs	Address		Output	Address		Internal Utility Relay
Low	0000		Motor	0500		1000
High	0001

Below is what the ladder diagram will actually look like. Notice that we are using an internal utility relay in this example. You can use the contacts of these relays as many times as required. Here they are used twice to simulate a relay with 2 sets of contacts. Remember, these relays DO NOT physically exist in the plc but rather they are bits in a register that you can use to SIMULATE a relay.

We should always remember that the most common reason for using PLCs in our applications is for replacing real-world relays. The internal utility relays make this action possible. It's impossible to indicate how many internal relays are included with each brand of plc. Some include 100's while other include 1000's while still others include 10's of 1000's! Typically, plc size (not physical size but rather I/O size) is the deciding factor. If we are using a micro-plc with a few I/O we don't need many internal relays. If however, we are using a large plc with 100's or 1000's of I/O we'll certainly need many more internal relays.

If ever there is a question as to whether or not the manufacturer supplies enough internal relays, consult their specification sheets. In all but the largest of large applications, the supplied amount should be MORE than enough.