When you're using any PLC module, like other electronic devices that you have at home,
you first need to apply power to that module to turn it on.
That means, for instance, if you have 5 modules in your PLC, you need to power each and every
one of these modules to turn them on.
The power wires are also connected the PLC input and output modules using the front connector.
That means when you want to power a PLC input or output module you first wire the power
to the front connector and then when you install the connector on the module, you'd be able
to power that PLC module.
So because of that, when you look at the wires on the front connector you'll see two types
of wires: 1.
The wires coming from the different devices in the field
2.
The power wires that are used to provide power for that specific module
For instance, for the module that I have here, these yellow wires are the analog control
signals coming from 2-wire pressure transmitters in the field.
This red wire here, that is connected to the first terminal of the front connector, is
the positive 24 volts DC power and this blue wire here, is the negative 24 volts power.
Now where do these power wires come from?
They are coming from a set of 24 volt DC power supplies that is installed in the same enclosure.
Now you may ask "how do you know which terminals you should connect the power wires to on the
front connector"?
Well depending on the type of PLC module you're using, you may wire the power signal to different
terminals on the front connector.
To figure out where you should connect the power wires you need two things:
1.
You need to have the manual of the PLC module that you're using
2.
You need to know which device you're trying to connect to this PLC module.
Because for different devices you may have different wiring.
For instance, wiring a 2-wire transmitter to the analog input module that I have here
is different than wiring a 4-wire transmitter to the same PLC module.
The manual can easily be found on the internet by doing a quick Google search for the order
number of the module that you have.
For instance, the order number for the analog input module that I have here is 6ES7 331-7KF02-0AB0.
I will Google this order number and here is the manual I'm looking for.
This manual includes detailed information about most of the S7-300 PLC modules but for
now I just need the info for this specific module.
So I'll hit control+ F on my keyboard, type in or paste the module order number and then
hit enter.
Here in the table of contents it says I should go to this page for more info.
By clicking here it takes me to the exact page I want.
As I mentioned the sensor that I'm wiring to this module is a "2-wire transmitter".
So I'll scroll down a bit and here is the wiring diagram that I'm looking for.
As you can see, here it says, to turn on this module I need to wire positive 24 volts DC
to the terminal numbers 1 and 10 of the front connector.
I should also wire negative 24 volts to the terminal numbers 11 and 20.
So this is how I find out which terminal on the front connector I should wire the power
to in order to supply the power for the module.
This diagram also clearly shows how to wire different types of transmitters to the front
connector.
Each one of these rectangles here represents a 2-wire transmitter.
As you can see, here it says if you have a 2-wire transmitter, which is what I'm currently
using, you can simply wire the positive to terminal number 2 and the negative to the
next terminal.
It’s wired the same for rest of 2-wire transmitters.
Now what if I want to wire a 4-wire transmitter to this module?
Here it also clearly shows how you can wire a 4-wire transmitter to this module.
As you can see, in this case, you just need to wire the positive and negative wires to
the front connector but here you also need to use an external power source to power the
transmitter as well.
So now you know how to power this analog input module and also how to wire 2 or 4-wire transmitters
to these module.
The other thing that you need to consider when you're wiring your devices to PLC input
and output modules is to make sure that you have picked the right type of front connector.
The front connector that you use may vary based on the type of input or output modules
that you have.
For S7-300 PLCs we usually have two types of front connectors: 20 pin front connectors
and 40 pin front connectors.
What I'm using here for this analog input module is a 20 pin front connector but as
you can see I have used a 40-pin type for this digital input module here.
So the type of front connector you're using will depend on the type of input or output
module that you have in your PLC.
What have you learned from watching this video?
Can you imagine implementing something from this video in your next PLC programming project?
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