Getting physical with industrial ethernet

Aug. 1, 2003
The old gag goes that, “The best thing about designing your products to standards is that you can choose from so many.” Even in the case of the most long-lived technologies, worldwide convergence to a single standard does not occur once multiple standards become entrenched in geographic or vertical market segments.

For example, a voltage, a frequency and a physical connector characterize electric power delivery. Yet after about 120 years of use, none of these variables have been standardized worldwide. Such is the staying power of an entrenched local standard.

The key component of the global convergence commonly called “Industrial Ethernet” is not really Ethernet per se, but rather the Internet Protocol (IP) running at the network layer, or layer 3 in the ISO 7-layer stack. IP has run on top of a huge variety of different physical and data link layers, or layers 1 and 2, respectively. Many different Ethernet physical layers have been specified since the IEEE first standardized the 802.3 network in 1985.

The evolution of these layers from 10 MB to 10 GB per second, combined with the advent of full-duplex Ethernet switches, has propelled Ethernet into many different environments and applications. In office environments, the ubiquitous implementation is an EIA/TIA Category 5E unshielded twisted four-pair cable with two pairs terminated on an RJ-45 connector. Network gurus call this configuration 10/100BaseTX, but everyone else just calls it Ethernet.

A wide variety of industrial environments exist, however, and in many of them, the Ethernet RJ-45 connector is not an acceptable choice. The reasons include the presence of chemicals, vibration, dust, strong electromagnetic fields, moisture, washdown of equipment and the overall higher probability of mechanical failure of the physical connector due to either contact with passing plant traffic or rough handling by personnel.

The solutions devised to remedy RJ-45’s limitations in the industrial environment separate into two major classes: adaptations that ruggedize the RJ-45 by encapsulation in an industrial housing; and replacement of the RJ-45 with an entirely different and fully industrial connector. Hardened RJ-45 connectors are physically larger. Their greater size is a liability in some cases, and not all commercial housing designs are compatible. As with any such solution, they do not address all issues in the most difficult service environments.

Replacement connector classes are based on the M12 form factor. M12 specifies a connector form factor only, not a complete connector design. It is compact and familiar to manufacturers in many industries where it has a long history as a technology used for connecting sensors and auxiliary power. It is well suited for difficult environments such as IP67. Besides its incompatibility with commercial off the shelf (COTS) components, there are other drawbacks to M12. Employment of M12 for Ethernet has created an incompatibility based on the use of four-pin vs. eight-pin M12 connectors. The four-pin connectors are more prevalent in Europe and correspond to the European practice of employing a two-pair cable in Ethernet service instead of the four-pair cable specified by TIA, the US-based telecommunications standards body.

Some proponents of encapsulated RJ-45 have also expressed concerns about adoption of the entirely different M12 technology at the 100MB signaling speeds commonly employed today. They contend that a 100MB Ethernet channel has a higher probability of developing electromagnetic problems when it contains non-COTS components, which have a significantly smaller experience base in 100MB Ethernet communication, and is operated for long periods in a harsh and electrically noisy industrial environment. The enormous difficulty of diagnosing cable problems in aging network infrastructures disposes them to prefer encapsulated COTS whenever possible.

Pay careful attention to the standards used at the physical layer of your industrial Ethernet networks, including connectors. Expect to find both M12 and ruggedized RJ-45 connectors on the market for the foreseeable future.

Harry Forbes, [email protected]

Harry Forbes is senior analyst for industrial networks and wireless solutions for ARC Advisory Group.

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