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.”
A contact is a computer code that monitors the status of an inputÉ ÉA coil is a computer code that monitors the status of an output. 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!”
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.”
A contact is a computer code that monitors the status of an inputÉ ÉA coil is a computer code that monitors the status of an output. 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!”
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