Though a great deal of the conversation on industrial networking of late has been on wireless technologies, copper and fiber are still the workhorses of industrial networks, especially with the rapid growth in deployment of Ethernet-based digital networks.
Because of the high bandwidth of 100 MB/s associated with Ethernet, it's susceptible to noise and, therefore, typically limited to the distances for which Cat 5e or Cat 6 cable can be used. This is in addition to the 100-m limit between nodes for Ethernet in general. Since copper communications systems let many devices share the same cable and communicate with each other, while fiber signal transmissions and reception are point-to-point, there will continue to be a need to mix systems for many years to come. Fiber typically can be installed 2 km to 20 km between nodes and is, therefore, well-suited to connect the various automation nodes throughout a facility — part of the reason for its increasing use in industrial settings.
SEE ALSO: Fiberoptics Extend Ethernet's Empire
Therefore, it's time to have a look at just what to consider when using fiber in the industrial environment.
Fiber Is Good for the System
Fiber optics, as the name implies, uses light rather than current or voltage for signalling. As such, the primary reasons to use fiber optics, other than for the greater distances possible, are:
• Ground Isolation — Since electrical currents do not flow on fiber-optic cables, grounding systems are not needed
• Noise Immunity — Because fiber-optic cables are immune to electromagnetic noise from radio stations, motor turn-on surges, welding discharges, electrostatic discharges and other radio frequency interference (RFI), and since fiber-optic cables do not conduct electricity, they can be placed on the same cable trays as power-carrying cables.
In the accompanying sidebar, you'll find an example of how fiber networks are being used to advantage in an electrically noisy environment in alternative energy projects using EtherCAT technology.
Mind the Details
Although fiber-optic cable is not susceptible to RFI, that does not mean that fiber-optic data communications are error-free. Though the cable itself might not be conductive, an armoured sheath, if used, could be conductive, as will any metallic hardware used in the cabling systems, such as wall-mounted termination boxes, racks and patch panels, which all must be grounded. In addition, the electrical code requires that all premises cables shall be listed and have flammability ratings per NEC 770.50 (2002), now 770.113 (2005). "Listing, Marking and Installation of Optical Fiber Cables" requires all cables within a facility to be labelled with the following exceptions:
Exception 1: Optical fiber cables shall not be required to be listed and marked where the length of the cable within the building, measured from its point of entrance, does not exceed 15 m (50 ft), and the cable enters the building from the outside and is terminated in an enclosure.
Exception 2: Non-conductive optical fiber cables shall not be required to be listed and marked where the cable enters the building from the outside and is run in a raceway (which includes a conduit) installed in compliance with Chapter 3.
Color Codes and Splice Tips
The TIA 568 standard specifies color codes for the individual fiber connectors: orange, black or gray have been multimode, and yellow has been single-mode. However, the advent of metallic connectors, such as the FC and ST, made color-coding difficult, so colored boots were often used. The TIA 568 color code for connector bodies and/or boots is beige for multimode fiber; blue for single-mode fiber; and green for APC (angled) connectors. In addition, color codes for the individual fiber sheaths in a multimode fiber are specified by TIA/EIA 598-A.
TIA/EIA 568 also specifies splice performance of 0.3-dB loss for both multimode and single-mode splices as industry-acceptable limits, and single-mode fusion splices are typically under 0.1 dB. The splicing itself can be a fusion splice that "welds" the two fibers together, usually in an electric arc. Fusion splicers are generally automated to produce splices that have minimal losses. A note of caution for the industrial setting is that, since fusion splicing uses an electric arc, it should not be performed in a dusty or explosive atmosphere, as the electric arc could cause an explosion or fire. The second and more common field splice is a mechanical splice that aligns the two fibers in a ferrule or v-groove, and then uses an index-matching gel or adhesive between the fibers to reduce loss and back reflection.