A little more about the control program

We talked a little bit earlier about the control program. The control program is a software program in the PLC’s memory. It’s what puts the control in a programmable controller.

The user or the system designer is usually the one who develops the control program. The control program is made up of things called instructions. Instructions are, in essence, little computer codes that make the inputs and outputs do what you want in order to get the result you need.
There are all different kinds of instructions and they can make a PLC do just about anything (add and subtract data, time and count events, compare information, etc.). All you have to do is program the instructions in the proper order and make sure that they are telling the right devices what to do and voila!…you have a PLC-controlled system. And remember, changing the system is a snap. If you want the system to act differently, just change the instructions in the control program.
Different PLCs offer different kinds of instructions. That’s part of what makes each type of PLC unique. However, all PLCs use two basic types of instructions:
• contacts
• coils

Contacts are instructions that refer to the input conditions to the control program—that is, to the information supplied by the input field devices. Each contact in the control program monitors a certain field device. The contact waits for the input to do something in particular (e.g., turn on, turn off, etc.—this all depends on what type of contact it is). Then, the contact tells the PLC’s control program, “The input device just did what it’s supposed to do. You’d better check to
see if this is supposed to affect any of the output devices.”

Coils are instructions that refer to the outputs of the control program— that is, to what each particular output device is supposed to do in the system. Like a contact, each coil also monitors a certain field device. However, unlike a contact, which monitors the field device and then tells the PLC what to do, a coil monitors the PLC control program and then tells the field device what to do. It tells the output device, “Hey, the PLC just told me that the switch turned on. That means that you’re supposed to turn on now. So let’s go!”

EXAMPLE

Let's talk again about that souped-up switching circuit, in which a wall switch and an overhead light are connected to a PLC. Let's say that turning on the switch is supposed to turn on the light. In this situation, the PLC's control program would contain a contact that examines the input device-the wall switch-for an on condition and a coil that references the light. When the switch turns on, the contact will "energize," meaning that it will tell the PLC that the condition it's been looking for has happened. The PLC will relay this information to the coil instruction by energizing it. This will let the coil know that it needs to tell its referenced output-the light-to turn on.

In PLC talk, this three-step process of monitoring the inputs, executing the PLC control program, and changing the status of the outputs accordingly is called the scan.