How field devices are wired to PLC?



There are two modules input modules and output modules in PLC. Each one of these modules are connected to different field devices. Such as temperature or pressure transmitters or switches, valves etc;.

fig 1

Look at the input or output component of the Siemens PLC as shown in the image above.

First we need get power supply to the each of the above modules.

fig 2

The power wires are usually colored in red for positive and blue for negative.

Opening the modules, we can’t directly connect the signal wires and power connection directly to the modules. We need to have a Front connector to make the connection.

fig 3

The Front connector just as shown above,it is just like we can do the wiring and fix it into the module.
Positive powered red line places at the top of the connection and the blue negative at the bottom.
Yellow colored wires are signals from field instruments.


The power lines coming from 24V DC power supply.

fig 5

The connection wires can’t connect directly to the front connector socket, so we connect wire ferrules at the end of each wire. As shown in above figure.

Now we have to complete the wire connections of field device .Before that we have to know that there are specific ports for different transducers in the PLC input module.
Depending on the PLC module we are using we may needed connect the PLC at different locations.
We need to have the manual and need to know the device which is connected the PLC, to figure out the where the power should connect. For a particular device there will be a dedicated port.

From the PLC’s manual we have to select the field device, the port allocation would be explained there.

Need of our purpose we can choose the 20 pin or 40 pin front connectors

fig 6