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1 April 2005

SCADA, RTU protocols

A large part of any complex SCADA system design is involved with matching the protocol and communication parameters between connecting devices.

There are about 200 such real-time user layer and application protocols. These include both proprietary and non-proprietary protocols, some of which are:

• Allen Bradley DF1,DH, and DH+
• GE Fanuc
• Siemens SINAUT
• Mitsubishi
• Modbus RTU/ASCII
• Omron
• Toshiba
• Westinghouse

The industry is now moving away from many of the old and proprietary protocols. These RTU/ PLC protocols are emerging as virtual standards in modern SCADA systems.

Modbus: The point-to-point Modbus protocol has become a virtual standard for RTU and PLC communications. During communication on a Modbus network, the protocol determines how each controller will know device address, recognize a message addressed to it, determine the action to be taken, and extract any information or data attached.

Modbus is a de facto standard in spite of some shortcomings. It cannot, for instance, handle large positive and negative numbers. This has resulted in a number of companies' specific expansions of the protocol, such as Bristol, Daniels, and ENRON. All PLC manufacturers have used the idea behind Modbus—a command set operating on 16 bit registers.

Modbus X : The non-proprietary Modbus X expansion has won favor and now works for a number of companies and utilities. SCADA software suppliers use it too. It fixes the Modbus shortcomings, makes it man readable, and makes it able to handle positive and negative numbers, with up to nine digits of resolution and an exponent range from -99 to +99.

Point protocol, designed to read and write to individual input/output points in PLCs on a factory floor. The Modbus X expansion of the protocol is a universal, non-proprietary expansion, which permits handling large process variables in plain ASCII with sign and exponent, capabilities that are missing in Modbus. With the universal Modbus X expanded protocol, it is no longer necessary to experiment with different proprietary expansions of the protocol.

Distributed network protocol (DNP) : DNP is a member restricted protocol used in some electric power systems. The DNP protocol has gone through various iterations. Presently, it is up to version 3.0. The DNP association has rules, which tend to restrict the use of the protocol, and major SCADA software suppliers have been slow in implementing the protocol.

American Standard Code for Information Interchange (ASCII) : ASCII is the dominating computer protocol. Virtually all computers, printers, and modems and many sensors, actuators, and flow computers now communicate in ASCII.

IEEE 60870 : This protocol works mostly in power transmission and distribution systems. IEC 60870-5-101 is an International Communications Protocol Standard for the Telecontrol of Electric Power transmission systems, which is being widely adopted in many countries throughout the world.

The local area networks/protocols from sensors/field devices to the PLC/RTU and from PLC/RTU to SCADA are:

Sensor networks : These are basic on/off field devices connecting networks.

Fieldbus networks : These connect analog and smart field devices such as valve actuators, pumps, and other field control systems.

Control networks : These are for peer-to-peer connections between control systems such as SCADA/DCS/Analyzer/Safety PLC systems.

Safety buses : These are for deterministic time sensitive safety type device connections.

With these open type industry standard emerging protocols and networks, availability of drivers and programming software can exist from many, if not all, SCADA system vendors.

Nicholas Sheble (nsheble@isa.org) edits the Networking & Communications department. Rao Kalapatapu (vrkalapatapu@hotmail.com) is an ISA member and a professional engineer. He designs and installs SCADA, DCS, PLC, and other control systems for oil and gas pipelines and chemical and petrochemical projects. Read his paper SCADA protocols and communication trends at www.isa.org/intech/April2005/NetCommDept.