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The operation of the PLC from a technician’s point of view is reasonably simple. PLC’s were designed to be simple from the start.  I have found that the biggest impediment to being a good technician or programmer is that many people refuse to believe that PLCs are simple!!  Even the wiring of a PLC is simplified.  Take the 3 wire control circuit as an example.

In hard wiring we have to do “Plumbing for Electrons.”  Everything is based on the Flow of Electrons to get things done.  Therefore, all the components, Start, Stop buttons, Hold in contacts, Coils and Overloads, all have to be interconnected based on making or breaking the flow of current.

This can get complicated and involves the use of stacked contact blocks on buttons, Control Relays, miles of wire, and hours of time both building the system, and troubleshooting.   This was necessary before the advent of modern electronics.

Even though most of todays PLC’s still use a programing language called Ladder Logic, and some people still talk about “Power Flow”, when describing what happens on the programming screen, there really is very little “Power Flow” happening in a PLC.

On the input side, the only current that flows is from the Supply, through the field device, into the detection circuit, and then back out of the module to ground, or back to the Power Supply.

None of the components is wired for Current flow from one device to the other.  Everything is really set up to tell the PLC whether there is power ON at the terminal or if the power is OFF at the terminal.  The PLC has no idea what is wired to it, and, could care less.  It’s only job is to notice if the Input terminal of the Input is ON or OFF.  Pretty simple.

The Current Flow in the Output side is also very simple. The Supply voltage that we put on each output module, is simply switched to the correct Output terminal based on the logic in the program.


Output Module Current Flow

The current that is needed to activate the Output does not come from the Input Field devices.  The only Current Flow on the Output side comes from the Supply terminal on the module, through the switching circuit on the Output, (Relay, Transistor, Triac, etc) then out from the individual Output terminal to the field device, then back to the Supply.

This is really handy.  It allows us to use a different voltage level and type on the Input side, versus the Output side.  For example, we could use 24V DC for the inputs that the operator has to touch, and 120V AC for the Field devices the operator never touches.  Try that in a hardwired system, without the use of a Control relay.

From many different points of view, this simplification of wiring is important.  It reduces the time to construct (physically install devices, wire multiple contacts, and devices) which reduces cost.  It also makes troubleshooting much faster since we only have one set of contacts on a button, as opposed to several, of which only one might be faulty.

Since PLCs also eliminate a large number of Control Relays, which can have a multitude of different types of failures, the system reliability goes up, and down time goes down.

At this point I usually get the same question from the students.  “If the Inputs and Outputs have no physical connection, how does anything get done?”

My usual response is “Welcome to Hogwarts”.  This is where the “Magic” of the PLC comes in. As we said before, in a hardwired system everything works on Current Flow.  In the world of the PLC, everything works on LOGIC FLOW.

I still hear instructors talk about power flow when dealing with a PLC program.  Since when does power (current) flow across a line on a computer screen.  If that were true, we would get a shock touching that line on the screen.  I have never seen that happen.

PLC programming is done using TRUE or FALSE paths.  If you want something to happen you must supply a TRUE PATH.  The default color of a TRUE path on most software is GREEN.  Green is related to TRUE, and not green is related to FALSE.  If we have a FALSE path, nothing is supposed to happen.

This applies to most programming languages and manufactures.

It is this True and False that is the heart of programming and troubleshooting of PLCs.  The Logic paths that we have in the program will connect the Input and the Outputs.

Above are two examples of the same logic using two programming languages.  Both of them connect an Input Device to an Output device.

What these lines of logic say is: If the Input device at Input module 2, terminal 3 is ON (has power at the terminal and Status light is ON), turn ON the Output wired to Output module 1, terminal 2.

For you Siemens guys: If the Input device wired to Input Byte 2, terminal 3 is ON, (has power at the terminal and Status light is ON), turn ON the Output device wired to Output Byte 1 terminal 2.

In the above example, with all the lines being black (not Green) we would know that the logic is NOT TRUE.  Input I 2.3 is NOT ON. Therefore, the Output device should be OFF!

In the above example, with all of the lines being Green we would know that the Logic has a TRUE PATH.  The Input IS ON so the logic has turned ON the Output.

This is how we can tie the Inputs to the Outputs without and physical connection.  Let’s look at this as a whole picture.

The logic in the Program says, that if Input I2.3 is on, turn on Output O1.2.  Pretty simple.  When the button wired to Input 2.3 is pushed, the terminal and Status light are ON.  This satisfies the conditions of the program!  Since the Conditions are satisfied, the program will turn ON Output O1.2.  What could be easier??

This is the basis of how a PLC system works.  Wire in all the devices that give information about what is happening in the real world to the PLC.  Write a program that tells the Outputs to come on under certain Input conditions.  Then turn ON the Outputs.

PLCs are easier to Troubleshoot, Wire and Program if you think Simple.