In a new light

Processing facilities are beginning to realize the benefits of using fieldbus networks for predictive maintenance and operational cost savings. Enhanced asset management is achieved by allowing devices on a network to report their status and continually monitor the health of an entire system. Many of these facilities use fieldbus networks such as DeviceNet or Profibus to communicate data from r...

By Todd Shadle, Phoenix Contact September 1, 2009

Processing facilities are beginning to realize the benefits of using fieldbus networks for predictive maintenance and operational cost savings. Enhanced asset management is achieved by allowing devices on a network to report their status and continually monitor the health of an entire system. Many of these facilities use fieldbus networks such as DeviceNet or Profibus to communicate data from remote sites to the control center and back again. However, a traditional copper-based fieldbus system has many restrictions.

Converting a copper-based bus system completely or partially to fiber optics has many advantages, which include:

Immunity to electrical interference

Isolation within the network

Exceeding electrical restrictions such as maximum length, speed or topology

Increasing the reliability of an entire network infrastructure.

Immunity to electromagnetic, radio frequency interference

Fiber optic cables are available in several different materials. The type of material affects the cost, distance the cable can be installed and the amount of available bandwidth. Based on the application, an end-user could choose polymer fiber cable (plastic); hardened clad silica (HCS); multi-mode glass; or single-mode glass. All have at least one thing in common: They are not an electrical conductor. They are completely immune to EMI or RFI noise that could be induced on standard copper wire.

The immunity makes fiber perfect for electrically noisy industrial environments because it eliminates the need for grounding and shielding. This is also beneficial when multiple cables are installed side by side for long distances because there is no possibility of crosstalk. Fiber cable can be installed next to power lines, high-voltage tracks and on automotive manufacturing lines where robotic welding equipment is operating.

Immunity to overvoltages

As fiber optic cable is immune to electrical noise, it has a natural resistance to effects of overvoltages. Devices connected directly to fiber optic cable cannot be damaged by a lightning strike that occurs near the fiber. Although use of overvoltage protection components can be minimized in a fiber optic installation, power connections and copper cable connections should always be protected.

Complete potential separation

Grounding and shielding are often done at multiple points throughout a facility. An equalizing of currents — commonly referred to as ground loops — can exist between these different points of grounding. Ground loops can cause corrupt data and unnecessary packet repetition. Because they can be so intermittent, they are difficult to troubleshoot. The fiber optic cable isolates the path a ground loop will travel. Eliminating this path makes installation much easier than if standard data cables were used, since shielding and grounding are not necessary.

Increased medium performance

The decision to install a copper or fiber optic transmission system is based on several factors. Optical fiber is definitely the best decision if the application requires long distances and high bandwidth, which can be has high as several Terabits of data. This would require thousands of copper connections.

Another advantage is the ability to multiplex through the fiber cable, creating different channels or pathways for data to travel. This wavelength division multiplexing can create up to 80 separate channels. Additionally, the inherent low loss characteristics of fiber cable allow long distances without additional amplification or repeaters.

Electrical standard vs. protocol

There are many bus systems on the market using common electrical standards such as RS-485 and RS-422. A common misconception is that the protocol used in a bus network is the same as the electrical standard.

The electrical standard, or Recommended Standard (RS), defines the electrical characteristics of the bus, voltage requirements, network topology, whether it is a multi-drop or a multi-point networking, maximum speed and distance of the bus.

A protocol is a set of rules used within the data that is exchanged between two end devices or software applications. Protocols such as Profibus, Modbus or DeviceNet define these rules. The protocol includes information such as 2- or 4-wire technology, multi-master or master/slave bus, transmission speed and how the data will be coded and formatted. Understanding the bus system protocol is critical to successful installation of a fiber optic network.

Advanced fiber optic features

Fiber optic technology has seen many changes over the last 10 to 15 years. Copper to fiber-optic converters are now multi-functional devices. These modules will not only convert from copper to fiber; they include multiple fiber-optic ports to create redundant point-to-point configurations or full-scale fiber-optic networks using star, tree or redundant ring topologies, depending on the protocol.

Built-in local diagnostics or power meter capabilities use LEDs to indicate fiber-optic emitter strength between modules. Additional remote alarming capabilities are possible using a dry contact that will change state prior to losing communications. To accomplish this, the receiver continuously monitors the safety reserve threshold between two fiber-optic devices.

Electrical standards have some limitations, but some fiber-optic devices can overcome some of these. Multiple fiber-optic devices that share a common backplane and communications to multiple modules enable several different network configurations without sacrificing the data signal’s reliability.

With bit retiming and bit oversampling, networks can have cascaded levels with unique branches. Data signals can be up to 35% distorted on the input and regenerated before passing on to the next device. These advanced features enable complex network structures, availability and serviceability while improving overall system performance.

Understanding the basics

The simplest description of fiber optics is the conversion of electrical signals to light pulses transmitted through fiber-optic cable and then converted back to an electrical signal. In some configurations this electrical signal can be converted from one standard to another, such as RS-232 to RS-422.

For our discussion, we will assume the conversion from copper to fiber and back again keeps the same electrical standard from transmitter to receiver of the fiber converters. Some fiber-optic converters have either a fiber transmit or a fiber receive connection.

Others have both a fiber transmit and a fiber receive connection. Still others have two fiber transmit connections and two fiber receive connections. Different versions allow for network flexibility and different topologies.

Structure of optical fiber

Fiber-optic cable is designed and manufactured for many different applications. There is fiber cable for indoor, outdoor, underground and highly flexible operations such as on moving machines. A few principle parts of a fiber cable are the same regardless of what material is used to make the cable.

The core of a fiber is the actual path the light uses to move data. The cladding surrounding the core is an integral part of the fiber cable, as it has slightly different properties from the core and assists with light refraction. The final layer is called the coating. The coating has no refractive benefits, but it protects the cable from the outside environment. Aside from the actual cable, sometimes an aramid yarn is used for protection of each cable, and then an outer jacket.

Three types of material are commonly used to make fiber-optic cable: glass, plastic or a combination of the two, called hardened clad silica (HCS) or plastic clad silica (PCS). HCS or PCS are glass core fiber cables with a plastic/polymer coating.

Fiber cable is also available in different diameters. The diameter is measured in microns and contains a number for the core diameter and a number for the core and cladding diameter combined. Typical fiber sizes are 980/1000 micron, 200/230 micron, 50/125 micron and 9/125 micron. The fiber cable’s material and diameter work together at their lowest attenuation with an optical wavelength. Different fiber-optic cables are compatible with different wavelengths (See Fiber optic wavelengths table).

Fiber-optic cable is also differentiated by two types of propagation: multimode and single-mode.

Multimode fiber exhibits a propagation path of several modes (beams) of light traveling down a piece of fiber. It has a larger core and less cladding, creating more attenuation of the light and less overall distance. This is shown in the polymer fiber cable with the light beams refracting off the sides of the cladding.

Multimode fiber also can be constructed of HCS and glass. The smaller core diameter and larger cladding diameter create a more direct path for the light to travel down the fiber. This lessens the attenuation and allows greater distances.

Of course, the single-mode fiber has the smallest diameter core and largest percentage of cladding. This allows a single beam of light to travel down the fiber with the least amount of attenuation and greatest distances. Single-mode fiber is consistently made from glass.

A common question is, “Why use plastic fiber with a size of 980/1000 microns connected to a fiber transmitter using 660 nm wavelength?” This is based on windows of opportunity, or a window within the wavelength such as 660 nm that will provide the lowest attenuation in the cable and the maximum cable distance.

Why not just use single-mode fiber for all applications since it has the least attenuation, furthest distance? The cost factor — the equipment required to generate the laser emitter is much more expensive than a simple LED that can be used as an emitter for plastic fiber cable. The advancements in fiber-optic technology make fiber optics very competitive with copper-based wiring, especially on the plant floor.

Consider using the table and the POF column. If the pricing had a base of 1, the HCS/PCF at 660 nm column would be about 1.5 times the cost of POF. HCS/PCF at 850 nm would be twice the cost. Glass at 850 nm is 2.5 times the cost and the 1,300 nm columns are three to five times or greater the cost.

The logical decision is to choose the fiber-optic technology that best fits the needs of the application. It is definitely not a one size fits all solution.

Fiber optic wavelengths

100 m
800 m
2,800 m
4,800 m
25 km
45 km

The table shows the cable types, distances, wavelengths, mode and the typical size of the fiber core/cladding used in industrial applications.


Fiber mode

660 nm
660 nm
850 nm
850 nm
1,300 nm
1,300 nm

Size in microns

Fiber-optic cabling provides many advantages. Understanding the technology will assist in designing a cost-effective fieldbus network, as well as increasing the reliability and functionality of an entire network infrastructure.

Author Information

Todd Shadle is Product Marketing Lead Specialist at Phoenix Contact.

Automation Career Connection at ISA Expo

The International Society of Automation will focus on the growing issue of workforce development at its Automation Career Connection, part of the annual ISA Expo Oct. 6—8 in Houston. Three events make up Automation Career Connection: iAU2M8.09, YAPFEST, and the pilot Undergraduate Student Research Conference. Each event includes unique, educational activities designed for each age group. The fourth annual YAPFEST on Oct. 6 is a social networking party for ISA EXPO attendees ages 18—30 who want to learn about careers in automation.

“YAPFEST is a key opportunity for young people to mingle with leaders in automation, peers, and members of the society for automation professionals, ISA,” said Bernard Penney, director, marketing and communications, ISA. “Events like YAPFEST offer attendees an economical opportunity to make potential career connections and build a professional network in one location.”