When PLC’s first came out, the idea was to make them as usable as possible by “Electricians” and “Engineers”. In order to accomplish that in North America it was decided to use “Relay Ladder Style” programming.
This is a simple example of a hardwired Relay Ladder Control diagram. You can clearly see the and the symbols on the diagram. Ever since electrical was started, these two symbols have been called NORMALLY OPEN and NORMALLY CLOSED. They refer to the condition of relay contacts or Auxiliary contacts. Basically, any contact that is connected to a Coil that is used for control. The Open state meant that current would NOT flow so nothing happens. The Closed state WILL ALLOW current to flow so something happens. This was accomplished by using relays. Here is a Picture of the operation of a Relay
The Coil is turned on (Supplied with current) which energizes the Electromagnet. The magnet pulls the plunger up which causes the arm to move from one Contact to the other. When the Coil is turned off, the magnet turns off and the spring pulls the arm back down to the starting position. The Normally Closed contact is the one that is touching when the Coil (Magnet) is not energized. I often call that the “Out of the Box” state. The Normally Open contact, is the contact that is not touching when the Coil is not energized.
The terminology of Normally Open and Normally closed is also applied to many other electrical devices, like sensors, push buttons, limit switches, and Selector Switches. The concept of Normally Open (Not letting power flow) and Normally Closed (letting power flow) is extremely well entrenched in the DNA of Electrical people.
It therefore seems logical to use these same symbols when we want to write a PLC program. There is only one problem. PLC’s do not give a darn about power flow in a program. All of the control in a PLC is done in the CPU. Which is about the size of a pencil top eraser. It contains millions of transistors and diodes, BUT NOT ONE RELAY.
Transistors will turn ON and turn OFF, but they never open or close like a switch. As a matter of fact, they are always connected. That is why they are called SEMI CONDUCTORS.
Allen Bradley/Rockwell (A-B from here on out because it is less typing) has changed the names of these instructions in almost every new software that came out. When I started in 1979 they were called Examine ON and Examine OFF. Then came the SLC 500 software and they were called Examine if Open and Examine if Closed. Then the Control Logix calls them Examine ON and Examine OFF again.
Many manufacturers have used, and still use, these symbols in their software. I have found that over the years, using the term Examine ON and Examine OFF, and the theory that goes with that terminology, has worked 100%, with every make of PLC and micro controller I have seen.
The largest problem is that these symbols create a tendency for people to go and look at whether the input devices are either Normally Open/Closed. When we are hard wiring a system, we have to worry about CURRENT FLOW. (See the How a PLC Works Section) The Program of a PLC does not work on Current Flow. It works on Logic or True and False. How do we relate the Symbols EXAMINE ON and EXAMINE OFF to writing a program and the status of the Input or memory bits they are addressed to? Let’s take a look.
For a standard 3 wire control circuit, we would have the Start Button, the Stop Button, the Hold in Contact and the Overload Contact. They are wired to the Input module individually. They are wired so that the PLC can see the ON/ OFF state of each device separately.
The PLC PROGRAM decides on how the circuit works, NOT the way that we wire the circuit. The PLC needs to only look at each device individually. From a Hard Wired point of view, this wiring will never work!!
I will give you the program the way most people would write it when they first start out, and we will dissect how it works or doesn’t work.
Looks right doesn’t it. I have a Normally Closed Stop button, and Overload contact. Both the Start Button and the Hold In contact are Normally Open. When you download the program and try it, here is what you will get when you push the Start Button:
This is your first proof that PLCs do not work on Hard wired principles. They work on what they see coming from the inputs, and logic. Let’s refer back to the Input wiring diagram to see exactly what the PLC sees and how it interprets it.
The PLC has no idea what kind of device is wired to it! The only thing that the PLC knows for sure, is if that device is supplying power to the input terminal or not. The Status light tells us right away if there is voltage or not. The Status light is ON if there is power, and OFF if there is no Power.
In this case, with all the devices in their NORMAL state, which lights are on? The Stop button and the Overload contact are ON, because they are N.C. The Start button does not turn ON until we press it, and the Hold in contact does not come ON until the contactor has energized. Now let’s look at why the program didn’t work.
We used an symbol for the Stop button. In PLC language that Symbol is an EXAMINE (FOR AN) OFF. Is the status light for the Stop button OFF? No, it is not! Therefore, the instruction is FALSE! This alone would stop the pump from turning ON. But let’s look at the Overload contact. We used an EXAMINE (FOR AN) OFF there as well. Is the Overload Status light OFF? No, it is not! Therefore, the instruction is FALSE! We have a second FALSE keeping the pump off.
Going and looking at the kind of contact that is on the input device WILL NOT tell us what INSTURCTION TO USE. and are INSTRUCTIONS NOT CONTACT SYMBOLS. Let’s change the instructions to what they need to be and see what happens.
Once we change the instructions we see that both the Stop button and the Overload instructions are TRUE (Green). The Examine ON has found an ON condition for the Stop button and the Overload. The Start or the Hold In contract are looking for an ON condition but have not found one yet. Pushing the Start button will give next display.
When we push the Start button the PLC sees an ON there as well. It now has one TRUE path so it turns on the Output Pump. As soon as the Pump turns ON the Hold In contact will close giving us a True ON as well.
At least one complete path of TRUE instructions is necessary to turn on an Output Instruction.
You will find it easier to relate the Examine ON instruction to the condition of the Status Light rather than the actual contacts on the input device. The diagram shows the relation of the device to the ON/OFF state of the Status Light, and how the Status Light relates to the Instructions.
Examine ONs need the Status light to be ON to be TRUE.
The Examine OFF is looking for the Status Light to be OFF to give a TRUE path. Basically, TRUE paths make something happen, and FALSE paths keep something from happening.
As an example. How about a Red pilot light that goes on when the pump is ON and a Yellow Pilot light that goes on if the Pump is off. Here is the Program:
Notice that the second rung is TRUE because the Examine ON of Pump is TRUE. It is looking for an ON condition and it has found it. So it is TRUE. (The Pump is running.) The third rung is FALSE, because the Examine OFF is looking for the pump to be OFF in order for the instruction to be TRUE. The pump is on so the instruction is FALSE.
With the Pump now OFF, the Examine ON of Pump in the second rung is FALSE. This will turn OFF the Red Pilot Light. The Examine OFF of the Pump in the third rung is now TRUE (the Pump is OFF) therefore the Yellow Pilot light is on.
To summarize. With PLC’s we do not have to care about what type of device we are referencing, or if it is open or closed. That is only a concern in a hardwired control. Each device is wired to the PLC separately so the PLC can determine what device is ON or OFF individually. The ON OFF status of the device is all we care about. We can get this state easily from looking at the State of the Status light.