As Ethernet becomes increasingly used for industrial automation networking applications, it is natural to expect that the cabling best practices found through standards-based methods in the IT world will be used more and more on the shop floor. Although it is certainly true that cabling connections from the data center to telecommunications room to the zone enclosure and the control panel largely adhere to structured cabling practices used in the IT industry, it is not always the case with cabling that goes from the control panel to the machine being monitored and controlled. This is thought to have come about because of legacy cabling techniques used within the control panel industry in the days of prevalent use of proprietary fieldbus communications. These centered on the direct connection of cabling to terminals on pieces of equipment using screw terminals or soldered connections made at the time of installation.
As Ethernet made its way progressively to shop floor applications, the industry practice tended towards field attaching an RJ45 plug onto each end of two or four pair cable to directly connect two pieces of equipment requiring bi-directional Ethernet communications. As the use of Ethernet developed into areas requiring ingress protection of the connectors, for these connections to plant floor devices, sealed versions of the RJ45 became available, as did sealed 2 pair (M12 D-code) and 4 pair (M12 X-code) connectors. This type of cabling deployment is often termed point to point cabling and focuses on the use of stranded cable for flexibility and ease of installation.
Putting it very simply, structured copper cabling centers on using solid copper conductor cable terminated with RJ45 jacks. These jacks are mounted in a physically secure manner, for example by installing the jack into a patch panel, a wall mounted faceplate or a DIN Rail Mount Adapter . Flexible patch cords using stranded cable are then used to connect the jacks at each end to the corresponding pieces of equipment being connected. Patch cords are available with RJ45 and M12 connectors so can connect to devices in varied environments and with various data rates. Leaving out a number of details, the solid copper cable assembly forms the basis for the testable assembly and is called the permanent link. Any future moves, adds and changes would then be made through alternative end point connections in a patching field using patch cords. As the structured cabling standards have evolved, so have the test methods to support them to make sure the cabling meets performance requirements ensuring the integrity of Ethernet data transported over the channel. Today, the testing focuses on field measurements of the permanent link at the time of installation and users can use performance-compliant patch cords to ensure channel performance is met.
In point to point cabling deployments, I have seen that prevalent practice is to field terminate stranded cable with RJ45 or M12 plugs primarily between the control panel and the machine, and then connect directly to equipment. It is rare that the electrical performance of the assembly is measured to ensure that data can be reliably exchanged between connected equipment. Indeed, a user has to be very careful in how measurements would be made with these plug to plug field installed assemblies, since virtually the whole structured cabling industry has driven towards making reliable and high performance measurements of jack to jack cable assemblies. This practice (of not field testing) has not, so far, given rise to a significant number of problems in these shop floor applications. However, as data rates increase, the criticality of the simple field installed cable assembly becomes that much more important.
Although there are cases where point to point cabling may be advantageous to consider (and I’ll deal with some of those in a separate article), I would always recommend the deployment of structured cabling for uplinks and critical connections to ensure that the performance of the cabling system is measured and known in order to enable the highest confidence in industrial networking applications.
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