IP Communications in the Field Level: Is Cable Reuse an Option?

By Guadalupe Chalas | November 12, 2024

The reuse of serial communication cable for IP applications depends on various factors. In a series of comprehensive evaluations, Phoenix Contact set out to test various cables to determine the maximum length through which they could enable IP communications when used as an SPE physical layer and create guidelines for cable reuse.

Serial communications for field devices

For decades, serial communication protocols in the form of fieldbuses have been the bridge between automation and the devices in the field. Fieldbuses are used to connect sensors, actuators, valves, and other devices located in remote locations to the higher levels of the communication pyramid. Fieldbuses offer several advantages over analog communication methods as they can accommodate multiple signals on a single communication channel. Figure 1 shows a representation of the different levels in the automation pyramid where sensors and actuators rely on fieldbuses to push information onto the control level.

Figure 1: Levels of communication within factory automation.

Despite the many benefits of fieldbuses, there are shortcomings related to bandwidth and latency when compared to IP communications. The transmission speed of many fieldbuses is limited to Kilobit per second (Kb/s) versus Industrial Ethernet, which supports speeds above 100 megabits per second (Mb/s). While not all field devices require the speeds and latency enabled with IP communications, critical tasks can be optimized with increased performance.

Benefits and challenges of IP-enabled field devices

When considering Ethernet-enabled devices in the field, the cabling infrastructure requirements of different applications are very important. Factory and building applications may use copper Category cable, while process automation plants, which may contain hazardous environments, may use fiber optics infrastructure for high-speed data transmission. Once integrated into the network, the IP-enabled field devices can have an IP address and wide-open visibility inside the network. This visibility enables real-time communications without a gateway, higher bandwidths, and security features such as encryption and authentication.

Figure 2: With IP all the way to the field level, pushbuttons and cameras can be part of the same network.

One of the barriers to implementing Ethernet networks all the way to the field level is the amount of serial communication cabling infrastructure that already exists in buildings, factories, and process plants. Even if the installation of new Category cabling is an option, field devices in locations farther than 100 meters from the switch would either require repeaters to extend the copper signal, or fiber-optic infrastructure, which could be expensive and not perform as well in harsh environments with tight bends and high temperatures.

Using serial cabling for IP communications

New physical layers for Ethernet over copper aim to extend the reach of copper infrastructure to reach devices in remote locations. Now a part of IEEE 802.3, a standard that encompasses all physical layers for Ethernet deployment, single-pair Ethernet (SPE) physical layers cover different speeds and configurations for full-duplex and half-duplex communications over a single pair of wires (Figure 3). These physical layers for Ethernet communications are protocol agnostic, meaning they are compatible with industrial Ethernet protocols for factory, process, and building automation.

Figure 3: IEEE 802.3 standards for SPE and working groups.

SPE physical layers specify either a shielded or unshielded differential pair of conductors, a construction that matches cabling for traditional fieldbuses, such as RS485, CAN, Foundation Fieldbus, Profibus PA, BACnet, and others.

The infrastructure to deploy SPE on field devices is already available, with manufacturers continuing to release connectors and chip sets or PHYs. Switches are also available (Figure 4), including devices that support the Advanced Physical Layer (Ethernet APL) for process automation, and switches for 10BASE-T1L communications that support Power over Data Line (PoDL), enabling data transmission up to 10 Mb/s and power to the field device.

Figure 4: SPE sensors and field devices can be connected to control systems, asset management stations, engineering workstations, and the cloud.

The question of cable reuse has always been present in the context of SPE. The guidelines for the deployment of 10 Mb/s SPE with intrinsic safety for hazardous areas, or Ethernet APL in process automation, have cable requirements that allow for the reuse of Foundation Fieldbus and Profibus PA cables. Factory and building automation markets have yet to come up with specific guidelines for SPE over existing serial cabling. While the IEEE specification for the SPE physical layer applies to both markets, questions remain about the extent to which serial communication cables can be reused for long-reach IP communications. As a manufacturer, Phoenix Contact has set out to answer that question by testing various cables to determine the maximum length through which they could enable IP communications when used as an SPE physical layer.

Guidelines for the reuse of serial cabling for IP communications over a single-pair Ethernet physical layer

Cable reuse should not be attempted without thorough evaluation of the cable structures in question. Any cable that is considered for reuse should be tested to ensure insertion loss (IL), return loss (RL), and all parameters described in the specification are met, even if not attempting to deploy the full length of the standard. Additionally, the age and physical integrity of the cable should be taken into consideration to avoid discontinuities. As such, we proceed to investigate the extent to which some serial communication cables can be used for SPE transmission.

For this investigation, three different serial communication cables were tested. The serial cables were terminated using a field-wirable SPE T1 CIM SF connector (Figure 5). The cable characteristics were as follows:

Cable type YV 2×0.8/1.4, switching wire, also known as bell wire, unshielded.
Cable type J-Y(ST)Y 2x2x0.8, shielded, used to transmit KNX signals in buildings.
BACnet twisted pair Plenum Rated EIA-485 communication cable, shielded.

Figure 5: Field Wirable SPE Connector IEC 63171-2 used for terminating serial cabling to the Fluke DSX 8000 Prototype Adapter

One part of the testing process consisted of finding a way to account for realistic cabling arrangements, as some cable deployments may branch out into different areas. These branches could be open or unterminated. Two configurations were considered for testing, as shown in Figure 6. These configurations were tested in open and terminated positions to determine the influence of terminating the lines in the performance of the link length.

Figure 6: Realistic cable configurations for evaluating potential cable reuse.

The best performance was achieved when measuring over the 40-meter length and terminating the branch lines. This configuration yielded favorable results and indicated the configuration is adequate for SPE transmission. None of the tests performed with open or unterminated branch lines yielded favorable results, and as such, branches should be terminated when considering cable reuse.

The next step was to determine the maximum distance these cables could run SPE. Signal-to-Noise Ratio (SNR) measurements were used to determine the maximum possible range for the cables. The results of this test yielded the following distances:

Cable 1 (YV) achieved a maximum range of 910 m with an SNR value of 27.6 dB.
Cable 2 (J-Y(St)Y) achieved a maximum range of 420 m, with an SNR value of 30.1 dB.
Cable 3 (24/1 PR) achieved a maximum range of 700 m with an SNR value of 27.6 dB (Figure 7).

These measurements show that SPE transmission, while not at maximum length, can be achieved using serial communication cabling.

Figure 7: SNR (Signal-to-Noise Ratio) results for the different cables tested.

The reuse of serial communication cabling for IP applications is possible and depends on various factors. The condition of the cable should be examined to avoid discontinuities. The configuration of the cable plays an important role and should be evaluated with care: open branch lines are not recommended for cable reuse while terminated lines can be suitable. In any SPE deployment with existing cables, branch lines that are open should be terminated.

Thorough evaluation and qualification of existing serial cabling should be performed before considering cable reuse, as it’s the only way to ensure performance in an existing configuration. For short distance evaluations, the focus should be on the return loss as it is a good measure of the reflections or power loss. For long distance implementations, the focus should be on the attenuation of the signal, as our results show distance can be a limiting factor. This evaluation has shown that while serial communication may not perform to the full length of 1 km as specified within the standard for 10 Mb/s transmission using SPE (10BASE-T1L), transmission can still be achieved for distances shorter than one kilometer.

To learn more about cable solutions for SPE and other networking strategies, visit Phoenix Contact.