Interface a PLC and SCADA

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PLC and SCADA Interface : How it Works

PLC is hardware which has all the inputs and outputs (I/Os)connected to it from the field. After this wiring is done, the I/Os are connected through a program (called as Soft-wired, as there is no direct-physical relation between I/Os) . The program is written taking the process into consideration.

The program which is written in a PC is then downloaded into PLC. The PLC runs the program and controls the outputs accordingly.

SCADA, on the other hand, is not a hardware. It is a software (usually installed in a PC or laptop) where we get to design the whole process in any industry by using the tools provided in the software.

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The above image is the pictorial representation of the process taking place in the field. It gives the real time description of the process.

The I/Os are not connected to SCADA directly.

SCADA controls the outputs through PLC. The programming part is saved in the PLC.

The PLC and SCADA are connected through various protocols (serial port, MPI, PPI, etc.) depending on the manufacturer.

SCADA screen

The I/Os are linked in the SCADA through addresses of those in PLC. These PLC input-output addresses are termed as Tags in SCADA. Each input and output that is designed in SCADA is given a tag which resembles an address in PLC.

Thus, whenever the data at the address changes, it changes the data in that Tag linked to that address, in the SCADA screen as shown.

These are the various SCADA screens helping the control engineer to monitor the process of every section of the field from his control room.

Communication Protocols

PLC and SCADA talk to each other using Communication Protocols.

Humans have Language to communicate with each other. Electronic devices have communication protocol to communicate with each other. Think of Communication Protocol as a language of electronic devices. Both PLC and SCADA understands this language.

There are thousands of human languages. Not all languages are understood by every human. Similarly not all communication Protocols are understood by all electronic devices.

Widely used Protocols:

  • Serial Communication(RS232,RS422 and RS485)
    • Modbus RTU
    • Profibus
    • DeviceNet
    • CAN Open
    • Allen Bradley DF1
  • Ethernet Communication
    • Modbus TCP
    • Profinet
    • Ethernet/IP
    • OPC

and Other vendor specific bus networks

To understand this clearly, you will need to understand Open System Interconnect (OSI) model of computer networking. This will give you the understanding of different layers. Pick up any standard computer networking book like “Computer Networks” by Andrew Tanenbaum.

My answer in short will be the following. Communication between SCADA and PLC happens using Ethernet (IEEE 802.3) as the physical media. This communication happens similar to one that happens for Internet communication and thus utilizes TCP/IP protocol. And finally the communication between SCADA and PLC uses Modbus as the application layer protocol.

Now to make it more clear let me draw parallel between Internet communication and PLC/SCADA communication. In Internet communication, Ethernet is used as the physical media. We may also use Wi-Fi as physical media when using mobile devises. Internet communication also utilizes TCP/IP. On Internet, depending on your use you may use different application layer protocol like HTTP (for web surfing), FTP(for file transfer) or SMTP(for email communication). So Modbus used in SCADA/PLC is similar to HTTP in Internet communication.

Published by Zahid Hossain