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The Basics of SCADA Protocols and RTU Communication

Imagine the simplest possible SCADA system would be a single circuit that notifies you of one event. Imagine a fabrication machine that produces widgets. Every time the machine finishes a widget, it activates a switch. The switch turns on a light on a panel, which tells a human operator that a widget has been completed.

In our simple model of the widget fabricator, the "network" is just the wire leading from the switch to the panel light. In real life, you want to be able to monitor a lot of systems from a central location, so you need a communication network. This a special kind of network used to transport all the data collected from your sensors ultimately to some kind of operations center.

SCADA Protocols

Early SCADA network protocols communicated over radio, modem, or dedicated serial communication lines. Today the trend is to put SCADA data on Ethernet and IP over SONET. For security reasons, SCADA protocols are frequently restricted to closed LAN/WANs to avoid exposing sensitive data to the open Internet.

Real SCADA systems don't talk with just simple electrical signals, either. SCADA data is encoded in protocol format. Older SCADA systems depended on closed proprietary protocols, but today the trend is to open, standard protocols and protocol mediation.

Real SCADA systems don't talk with just simple electrical signals, either. SCADA data is encoded in protocol format. Older SCADA systems depended on closed proprietary protocols, but today the trend is to open, standard protocols and protocol mediation.

This kind of protocol allows devices from different vendors to communicate with one another. This means these manufacturers want to achieve superior compatibility when they design their equipment's functionality and capabilities.

Using an open standard protocol is a very important decision that leads to cost reduction and maximized flexibility. The open standard has many benefits over the proprietary protocol, such as:

Examples of open protocols include SNMP, DNP3 and Modbus.

SCADA Diagram

RTU Communication

Sensors and control relays are very simple electric field devices that can't generate or interpret communication protocols on their own. Therefore the remote terminal unit (RTU) is needed to provide an interface between the sensors (data acquisition) and the SCADA network. The RTU encodes sensor inputs into protocol format and forwards them to the SCADA Human Machine Interface (HMI) - also called master station.

The best practice is to have a SCADA software that supports what HMI communication protocol you have, such as DNP3 (distributed network protocol) and Modbus TCP/IP.

This exchange can be asynchronous with events reported by the RTU as they occur. It can also be exchanged using a command and response protocol that is generally controlled by the SCADA master.

If the RTU is designed with outputs, it can also receive control system commands in protocol format from the master and translate them to the appropriate physical or communication outputs.

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