PLCs and SCADA

Generally with industrial automation control systems, a PLC is commonly used as the electronic hardware device connecting to electrical equipment requiring controlling or monitoring.

Electrical equipment may be motors, actuators, lights or relays as outputs from the PLC and also PLC inputs such as switch inputs, temperatures, pressures, voltages or currents as analogue inputs. PLCs will have specifically designed programs for the task required, typically, where it will monitor equipment inputs as well as user input to then control equipment according to the stored PLC program.

PLCs are designed to be rugged and withstand the more extreme environments of industrial situations, where there may be high levels of electrical noise, thermal stress or other environmental contaminants. Essentially, a PLC is a small computer that has been programmed for a specific task, which requires little or no attention for many years or operation.

In order to know the current state of operation within a PLC system at any given moment, it can be connected to a computer system via a communications link. Often, the communications link is based on a standard protocol, such as Modbus for example. Modbus is these days a non-proprietry protocol.

Having such a connection to a computer system provides a means to remotely connect with the process within the PLC, and allowing the recording and display of process data.

The computer system must therefore have an application that can communicate with the PLC system to facilitate this. The computer system may be physically located close by or far away from the PLC which is generally located in an electrical switchboard where equipment is electrically terminated. The communications between the PLC and computer system is often over a hard-wire cable and is typically one of a few standards, such as Ethernet or optical fibre or asynchronous serial like RS-232 or TIA-485.

As mentioned, the computer system must have appropriate software installed which itself must be configured for the specified process and also for communications to function to connected PLC equipment.

This is the aim of QuadlogSCADA.