Optical fiber brings performance benefits to industrial networks

Since industrial networks are considered mission critical for many companies, they deserve a more in-depth look.

When discussing the use of fiber optics in premises networks, images of desktops, servers, printers, etc. come to mind. While the typical corporate LAN makes up the majority of the networking market, it is not all of it. In a previous column, I briefly addressed the use of fiber in industrial networks. Since industrial networks are considered mission critical for many companies, they deserve a more in-depth look. In particular, it is worthwhile to explore how fiber can be deployed effectively in these networks and what products are required to make them viable solutions.

To better understand how to leverage the benefits that optical fiber can bring to your network, you first need to have a basic understanding of what these networks are used for, how they are typically configured, and how the current trends in the industry support the deployment of fiber-based systems. Industrial networks perform three basic functions:

• Process control
• Data collection
• Process monitoring.

For process control, MIS managers generally use special-purpose computers-either using computers unique to the application equipment or by deploying more-versatile programmable logic controllers (PLCs). Process control involves reading and reacting to process variables in real time. Often the process variables may reside on a different computer or PLC and are read over a network. Timeliness and accuracy are very critical, so the network connection must guarantee both. In this application, optical fiber offers users extremely low error rates and very little downtime because of its low maintenance and high reliability.

Data collection involves reading sensors and switches often located in and around equipment that utilizes heavy amounts of electricity. This equipment generates large amounts of electrical noise that can interfere with copper wiring systems. The interfaces to these sensors need to provide immunity as well as accuracy. Optical fiber's immunity to emi/rfi interference makes it an ideal medium for these environments.

Process monitoring requires communication with a user, which can be done via a direct user interface on the controller or over a network connection. When communicating to the user over a network connection, large amounts of formatted data often need to be sent over long distances. Frequently, a process engineer monitors the processes from a remote location or office. Here, optical fiber offers network managers greater flexibility because of its ability to carry signals over longer distances than copper-based networks.

One final aspect to consider when evaluating industrial networks is how control programs are downloaded into control computers. These programs are often developed by manufacturing and process engineers and are downloaded via a network connection from the engineering office to equipment on the manufacturing floor. Here, fiber's tre mendous bandwidth easily supports error-free throughput of the data.

Unlike data networks, control computers are left unchanged for several years. Once a computer has been tuned to a particular process on a piece of equipment, there is seldom a need to upgrade or replace it. To do so often involves significant amounts of downtime in production, resulting in severe loss in capital. Since this equipment is often running continuously, it is important to understand how it interfaces with the control network.

According to the Keithley Measurement Needs Tracking Survey, reported in the February ྟ issue of InTech, 74% of the network buses used today in control networks have some form of serial communications including RS-232, RS-485, or RS-422. Thirty-eight percent of these networks utilize some form of Ethernet. There are several other industry-specific interfaces mentioned such as Fieldbus or DeviceNet, but they did not have significant penetration. So a discussion of fiber needs to focus primarily on serial interfaces and Ethernet.

A common trend in control networks is to integrate them into the rest of the corporate LAN. As businesses become more dependent on e-commerce and just-in-time manufacturing, this integration becomes critical. For example, a company may need to modify one of their processes based on specific requirements from a customer's order. This integration requires use of a common networking infrastructure.

Another benefit of fiber is that it enables a common cabling infrastructure. By utilizing fiber, network managers can optimize their cabling infrastructure and futureproof their networks with spare capacity that can be used for either the control network or data network, whichever requires it.

Given the needs of industrial networks and the benefits offered by optical fiber, it would seem that fiber is an obvious choice. Unfortunately, until recently, fiber has not been used extensively in these networks. This is because of the historically high cost of fiber-based solutions, as well as their perceived complexity. But neither of these issues is true anymore.

The easiest and most cost-effective way to deploy optical fiber in an industrial network is for the control computers to have fiber interfaces. At this time, however, most of these devices use 10Base-T interface connections. In these cases, it is fairly straightforward to convert from 10Base-T to 10Base-FL. As more network managers implement Ethernet, they will likely turn to fiber because of its distance capabilities-it is not uncommon for a production-control network to cover distances greater than the 100-m limitation of 10Base-T. For networks that are implementing Fast Ethernet, and where distances are less then 300 m, the pending 100Base-SX standard provides a low-cost solution for fiber connectivity.

Control computers that are not connected over Ethernet are typically using either RS-232 or RS-485 connections. In the case of RS-485, distances up to 4000 feet are supported at 19.2 kbits/sec. But they utilize twisted copper wire that is sensitive to emi/rfi interference. These interfaces can be converted to fiber; however, this may impact the network topology as multidrop busses are not possible over fiber. Instead, each device needs to be connected with a dedicated fiber back to a central point where the multidrop topology can be emulated.

Another factor to consider when designing or deploying fiber optics in these networks is the operating environment. Optical fiber and the associated electronics need to support temperature extremes from 0 to 60°C. Additionally, they may need additional safety ratings required by industrial process-control standards.

Steve Stange is chair of the Fiber Optics LAN section (FOLS) of the Telecommunications Industry Association (TIA) and senior product manager for Transition Networks (Minneapolis, MN). He can be reached via e-mail at [email protected].