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At our College we teach both the Rockwell PLC and the Siemens PLC.  We also teach both Ladder Logic and Function Block programming.  The first course they take in PLCs uses the Siemens S7 1200 PLC and Function Block Programming.  I do this because they are also just starting to do hardwired relay schematics.  With the   and the  symbols being used in both hardwired schematics that work on power flow, and in Ladder Logic Programming, where there is no power flow, it totally confuses the students.  By using the Function Block programming we introduce them to Logic Flow without the confusion of the Examine ON and OFF symbols.

The switch from Siemens to Rockwell in the second semester brings an unexpected challenge for the students as well.  At least it is unexpected to me.  They seem to feel that the I/O wiring principles they learned last semester, must have changed this semester.  They seem surprised that they haven’t.  I/O wiring principles are the same for every PLC and even the CPU based micro controllers.  Yes NEMA and IEC nomenclature is slightly different.  Siemens uses the letter M to signify the minus, or return to the power supply terminal, and Rockwell uses the COM for the same thing.  That should not be hard to figure out if you understand the way that Inputs and Outputs work.

While there is no current (power) flow actually happening in the PROGRAM, even if you are using Ladder Logic, there has to be current flow in the Real World components. Not much happens in the real world without current flow.  So lets look at how this works.  THIS APPLIES TO ALL PLCs.  This might be a good place to talk about what is an input and an output.











Anything that is an Input brings information to the PLC.  These devices get wired to the INPUT terminals, either digital (ON/OFF) or Analog (Changing values, Temperature, Pressure, Flow etc).  Anything that gets told what to do by the PLC is an Output and is wired to the Output modules, either digital or analog.  Because these devices are in the real world they need real current flow to work.  Let us look at what that means.  We will do the Inputs first.

Inputs act like a Volt meter.  They have to be able to measure a voltage.  Every meter has two leads for a reason.  Every reading is really a comparison.  You can not compare just one item.  You can not compare AN apple.  You need something else to reference it against, like an orange.  The meter and the Input module work the same way.  You have the BLACK lead which is the REFERENCE OR COMMON lead.  This establishes the first thing to compare.  Then you have the RED lead.  This establishes the second part of the comparison.  The reading that you get, is the difference between the reference lead (Black), and the Probe Lead (Red).  The reading is either positive or negative, depending on the probe lead (Red) being at a higher voltage (more positive), or lower (more negative) than the reference lead (Black).

The comparison on the Input module is made between each input terminal and the Common or M reference terminal.  If the Input terminal is higher than the reference then the input is considered ON.  If the voltage is the same, it is considered OFF.  The Input does not care about the actual voltage.  It must be higher than a certain point which is dependent on the modules voltage range.

As a rule you would bring the positive side of the power supply to one side of each input device, and the other side to one of the Input terminals.  Then you have to wire the Reference terminal to the negative of the power supply (This can be reversed for certain electronic input devices. ie Prox sensors, encoders, etc) If you first power up the PLC, you should see the status lights of your N.C. devices already on.  If you get no status lights, or response from your inputs in the program, CHECK TO MAKE SURE YOU HAVE WIRED THE REFERENCE TERMINAL BACK TO THE POWER SUPPLY!!!

Outputs are similar.  The Output modules are only acting as a switch.  They switch power flow to each device as the program tells it to.  Inside most Output modules the 1L or V+ the Supply terminal MUST be wired to the supply voltage you want to use.  Inside the Supply will be connected to a number of individual Outputs.  When the Program tells the Output to turn on, the Output device (transistor, Triac, Relay, etc) is activated, and switches the supply out to that device.  The current flows through the device and back to the 0 Volts on the Power Supply.

We have to wire the desired Supply voltage to each Output module, then each individual output terminal is wired to one side of the device.  The other side of the device is wired back to the Power supply so actual current can flow through the device and it will work.  If your program works but nothing turns on in the real world YOU HAVE FORGOTTEN TO WIRE THE SUPPLY TO THE OUTPUT MODULE.

No matter the make of PLC the I/O is wired this way.  Each manufacturer may call the Reference point and the Supply point on the I/O by a different names, but they are all the same function.