Building a PLC System
The PLC Circuit
Now that we have a handle on Relay Logic, let's rebuild our circuit using a Programmable Logic Controller (PLC) as the controller.
Once again, we will use the Motor, L1, START, and STOP switches. But this time, we will connect them to the PLC rather than a relay.
Everything that was handled by the relay last time will now be handled digitally using Ladder Logic.
The ladder logic is already set up this time. In future lessons, we will talk about how the ladder logic is created.
For now, we will focus on configuring our modules in the PLC software and connecting field devices to the modules. We will also explain how to test the system.
We will also look at where you the programmer fit into the PLC system. Let's get started!
Communicating with The PLC
The PLC controls the system by making decisions for it. But you, as the programmer, need to tell it how to make these decisions.
That's the whole purpose of Ladder Logic!
Communication with the PLC doesn't happen in a vacuum. In fact, there are a number of inputs, outputs, and communication streams happening with the PLC.
The flowchart below lays out how the PLC receives input signals from input field devices, communicates internally and externally, and sends output signals to the output field devices.
It's all part of a system.
The PLC System
In a PLC control system, information flows into the PLC as Inputs or out of the PLC as Outputs.
Communication happens:
- Between the PLC and programmer
- Between the PLC and the input or output (I/O) field devices.
- Within the PLC (between the CPU, I/O modules, and memory)
The PLC is powered by an internal power supply (connected to external power) while the modules and field devices are powered by external power supplies.
The PLC Control System
Input field devices, like push buttons, send Input signals which the PLC receives via terminals on the Input modules.
Based on system instructions, the PLC transmits output signals to output field devices, like the motor and standby light, via terminals on the output module.
The PLC prefers to handle the majority of communication, which happens automatically between the PLC and I/O field devices. The communication flow can be one-way or two-way.
As the programmer, you're mainly responsible for initial programming, monitoring, and sending modified instructions to the PLC.
Multiple Power Sources are needed to run the system. The PLC has its own power supply that draws from an external source and prepares the precise voltages needed for the PLC modules. Other power supplies may be required for the field devices.
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