Fiber Optics in Industrial Applications

Ethernet has spread from traditional office environments to various fields, including harsh industrial environments such as factory and building automation. Today, copper cables are the most common option for Ethernet, but optical cables are gaining more and more success in industrial applications due to their long-distance capabilities and many advantages of galvanically isolated interfaces.

Packet loss or corruption can cause any network to fail, but it is more of a problem for industrial buses. These buses have some level of synchronization to maintain tight control and precision. Most industrial buses achieve synchronization through continuous and uninterrupted communication using specific time synchronization packets. When a packet does not arrive, it not only affects the synchronization of nodes moving down the link, but it also causes additional packet transmissions, resulting in packet loss or corruption.

The importance of mitigating packet loss and packet errors requires several considerations in industrial environments. Techniques for addressing issues arising from radiated and conducted emissions and the need for electrical isolation. Reducing the distance between network components can minimize the probability of packet loss and packet errors. Fiber optic cables have inherent properties that provide new ways to address these challenges and solve potential problems associated with link loss or Ethernet physical layer (PHY) impairments.

Radiated/Conducted Emissions

An industrial environment can include a variety of sources of electromagnetic radiation. Motors and mechanical equipment can generate parasitic signals that couple into copper cables and cause packet errors. These signals can interfere with the steady flow of information that industrial buses rely on. In the worst case, the cable can even become a path for high energy levels to reach the PHY, causing loss or even damage to the link.

Fiber optic cable can solve some of these problems. It does not have the electromagnetic properties that cause radiation to occur in copper cables. Fiber optic cables transmit light through plastic or glass. Because of the copper cable, there are no magnetic fields generated, and fiber optic cables are not susceptible to power surges and energy transient pulses in industrial environments.

Electrical isolation

In a fiber optic network, there is no electrical connection between nodes. Therefore, the network nodes cannot develop different common-mode voltage levels or common-mode transients on the cable, which could affect the data packets. Fiber optics may also be required if high voltages pose a safety concern.

When using copper cables, Ethernet network nodes are typically transformer coupled to the cable. While this does provide isolation, there is still an electrical connection between the network nodes. Fiber optic cables add another layer of isolation by eliminating any direct electrical path.

distance

The Institute of Electrical and Electronics Engineers (IEEE) 802.3 specification defines standard Ethernet as operating over 100 meters of copper cable. While this distance is sufficient for most applications, there is a fundamental limit to the distance that the physical copper medium can support without packet errors. As shown in Figure 1, the attenuation of Category 5 (CAT 5) cable limits certain applications.

Figure 1: Attenuation vs. frequency for Electronic Industries Alliance/Telecommunications Industry Association (EIA/TIA) Category 5 cable

Fiber optic communications support much greater distances. Whereas copper networks have node spacing of tens of meters, fiber optic networks can have node spacing of hundreds or thousands of meters. This allows for industrial applications where networks can be bridged between buildings or between substations connected to high-power grids.

Fiber optic communication will become an important part of industrial networks in the near future. It enables industrial Ethernet to have some very attractive characteristics, allowing users to operate in harsh environments and long distances, and has excellent electrical insulation properties.