Ethernet for the plant floor

What does 'Ethernet-enabled' mean? It means you plug it in, and that's it," begins Victor Wegelin, instructor of Industrial Data Communications, a training course (TS05) offered by the Instrument, Systems, and Automation Society (ISA). Wegelin emphasized the importance for plant engineers and others outside the IT department responsible for control system maintenance to understand the technology and issues involved in successful installation, commissioning, and operation of industrial control networks. Networks used exclusively for industrial control generally are used and maintained by people who are not communications specialists.

The seminar, held in June at the Hilton, Lisle, IL, explained the special requirements that industrial control networks have beyond business networks. According to Wegelin, industrial networks require predictable throughput and scheduling, extremely low downtime, and scalable size. These networks must be robust enough to operate reliably in harsh, and sometimes hazardous, environments. Most process network systems modify technology borrowed from business networks.

The bulk of the seminar centered on Ethernet and TCP/IP. Wegelin defined protocol as a "set of rules that determine how two devices should communicate." The tasks required of protocols are:

• Error detection and correction
• Routing messages through complex networks
• Data encryption and security
• Consistent signal levels among devices
• Network addressing.

There are many protocols. Trying to make sense of them is like eating alphabet soup. To organize them and understand how they interact, protocols typically are arranged into layers, grouping them by related tasks. For example, protocols that deal with the physics of message transmission, such as signal frequency, voltage level, and cable design, are grouped together.

Wegelin likened Ethernet to the U.S. Postal Service. Send the message in the proper format and protocol, and it gets to where it is supposed to go.

Protocols govern the way a letter is sent and received. The recipient receives and understands the message because proper protocols were followed. Not following appropriate protocols is like sending a message in a bottle — it may miraculously get to the intended recipient, but probably not.

In 1977, International Organization for Standardization (ISO) developed a 7-layer model — called the Open Systems Interconnect Reference Model (OSI/RM) — for organizing data communication protocols. These layers are as follows:

• Layer 1 = Physical
• Layer 2 = Data link
• Layer 3 = Network
• Layer 4 = Transport
• Layer 5 = Session
• Layer 6 = Presentation
• Layer 7 = Application

Wegelin discussed industrial protocol suites, such as Modbus, Modbus+, and AB DH+ as well as how they compare with commercial suites. He covered each layer of the model thoroughly, providing plant floor examples frequently. Wegelin said that it is important to understand that Ethernet is not a complete network on its own; it needs upper protocols.

Wegelin concluded the seminar with a presentation about OPC (opcfoundation.org). He said, "Until the industry begins to get together on the application layer, we still have problems," referring to the many fieldbus protocols available to the plant floor. According to Wegelin, OPC may finally offer the industrial control compatibility that could call a truce in the ongoing bus war.