How to integrate PLC into your Control System

Programmable Logic Controllers (PLC) finds common use in most industrial control applications. To be able to successful integrate PLC into the control and management of your plant; you need to know its features and capabilities.

PLC comes in many sizes; meaning various I/O and program capacities. Their capabilities vary greatly making generalizations difficult. The smallest sizes are typically referred to as nano PLC, micro PLC and mini PLC having fixed inputs and outputs. The mini PLC is used in small stand-alone applications.

The larger PLCs support redundancy for CPU, power supply and possibly the control network, but typically not for I/O cards although we have other large PLC that support I/O redundancy by using duplicate I/O subsystems with separate backplanes where the field instruments are wired in parallel to both I/O subsystems. The control network is typically a standard industrial Ethernet application protocol over Ethernet media and IP. For PLC the field cabling lands directly on the I/O card.

PLC usually supports very fast scan times as required in discrete manufacturing, especially the discrete logic, however PID loops add to the CPU load much more than discrete logic thus making the scan time slower. The scan time, although fast, may vary with task loading. Loops are not handled individually in a PLC. Any addition or change to the loop requires a download of the entire program which affects other loops in the CPU.

The PLC configuration software is separate from the HMI software as they generally come from different manufacturer, meaning we have two separate databases. The logic programming is first done in the PLC configuration software. Next the OPC server has to be configured. Note, a OPC server is a software program that converts the hardware communication protocol used by a PLC into the OPC protocol and the OPC client software is any program that needs to connect to the hardware such as Human Machine Interface (HMI). The OPC client uses the OPC server to get data from or send commands to the hardware.

For a native OPC server the configuration happens automatically but for a third-party OPC server manual data mapping is required which is time consuming and error prone requiring thorough testing hence native OPC server is preferred. Finally, the HMI database has to be configured for graphics, alarms, trend etc.

If the HMI software comes from the same manufacturer as the PLC, the intermediate data mapping step may not be required, making the integration easier, however, even if the PLC and HMI manufacturer are the same, the data mapping may still be required in case the two products were designed separately.

You can also read: 

• The Transition from Relays to PLCs Explained
• The Basics of Industrial Instrumentation and Control Systems
• Key Features of Distributed Control systems (DCS)

The Bottom Line

In most cases, each PLC manufacturer has a native protocol, yet open standard, which the PLC architecture is built around. It may be DeviceNet, Modbus, PROFIBUS, or CC-link etc. The PLC comes with its own native interface cards for the native protocol supported by the PLC manufacturer, but relies on third-party interface cards for other fieldbus protocols. The engineering software therefore automatically configures the communication interface card for the native protocol, but for other field protocols the interface card must be configured to map the variables before they can be used in the control strategy and graphics.

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