How Tough is Ethernet?

How Tough Is Ethernet?

Ethernet is becoming a regular in the specifications we receive from customers that require network connectivity in severe operating environments where we install our rolling mills. Is Ethernet a good idea in the first place? Can we take advantage of lower-cost commercial hardware, or are there applications where we should use more costly industrial grades?

Answers:

An advantage of Ethernet is that you can run more than one protocol over the same network. Fieldbus standards for Ethernet communication are maturing. We recommend a dedicated Ethernet subnet for the industrial fieldbus. Network switches provide determinism and avoid collisions. Switches should be used instead of hubs for industrial applications. Connector and cabling companies are developing industrial-grade Ethernet connectors and related hardware.

...Gateway devices for Ethernet are becoming increasingly popular. These devices map or interface other buses onto the Ethernet network, such as serial, CANbus, and a variety of fieldbuses. This is another reason why it is relatively easy to add Ethernet-based devices to existing measurement and control systems.

...Wireless Ethernet technology is widely available. It offers a rated range of up to 25 mi. (40 km). Wireless devices, fiberoptic links, and other bridges are transparent to the network, so no code changes need to be made to add wireless capability.

...For mission critical control loops, the software must match the hardware in terms of its suitability. The reliability of the embedded control system depends just as much on the embedded software and operating system (OS) as on the silicon. If the OS allows itself to be hijacked by instructions that are not mission-critical, then there is no way to guarantee that the system will meet its timing requirements.

...Under a general purpose OS, a control loop that normally runs fine on a general-purpose OS may fail when conditions change, such as when the application intermittently sends a packet of data over the Ethernet port. On the contrary, a real-time OS will prioritize, allowing the serial instructions to execute only when the time critical functions have finished and are "sleeping."

...Likewise, if an event occurs that requests the time-critical function, any lower priority operations are immediately suspended. This software pecking order allows the time critical task to exert absolute authority over the OS, preventing insubordination in the ranks over lower priority tasks. When the top dog barks, the pack is 100% dedicated to the request until it is finished.

...For lower priority functions, the operating system multitasks and gives each task a roughly equal piece of the remaining processor time. This combination of even-handed sharing of lower priority jobs and iron fisted dedication to the time-critical task means that as long as there is sufficient computational muscle available, a single processor can do it all. The same embedded system that controls the temperature, pressure, and fluid in a critical chemical manufacturing process can also serve up a web page and administer a file transfer protocol (FTP) server. Those normal priority tasks run automatically in the background when the critical loop is resting, but when the time critical loop awakes and snatches back the processor, it's got control within microseconds.

Ethernet is winning the protocol wars, even in the factory, because it has proven to be capable of delivering the determinism, speed, and reliability required by the industrial space. It also delivers new levels of productivity by enabling manufacturing enterprises to become truly integrated (top-floor to shop-floor). Ethernet wins on the basis of cost because it has long been the office standard for open communications, the components are commercial off-the-shelf (COTS) brands, and the skill-sets needed to install and maintain the technology are more widely available. Also, because of the widespread use of Ethernet technology, the rate of new developments and advances in this area is accelerated compared to proprietary communication protocols.

...Still, tough environments like steel mills do require tough solutions, and Ethernet is capable of meeting the challenges of these applications. Given the electrical noise and temperature extremes prevelant in these types of environments, fiberoptic cabling should definitely be a consideration. Copper cabling and wireless are often not viable options in applications where significant electrical noise is generated by large motors and drives. In addition to making the right cable choices based on the environment, it is also important to look at the specific connector requirements for each area of a given application.

...Regarding other network components such as switches, the decision to use industrial grade versus office grade again depends on the environment where each specific component will be located. For example, if a switch is housed in an environmentally controlled enclosure or control room, office-grade may be sufficient. However, if the switch is going to be located in the same cabinet as control equipment or in other areas where there can be a lot of noise, industrial grade equipment may be required. An advantage with Ethernet is that you now have more options--you can take advantage of lower-cost office-grade equipment where appropriate and use the industrial grade components only where necessary.