2026-04-20 Wiki, HVAC/R
Single Pair Ethernet (SPE) as a possible successor to Modbus RTU
Modbus RTU via RS-485 has been the established communication standard in refrigeration, air conditioning, and heat pump technology for decades. However, increasing demands on networking, data availability, and cyber security are increasingly calling this proven system into question. Single Pair Ethernet (SPE) promises end-to-end IP communication right down to the field device, higher bandwidths, and power supply via the data line. This article highlights the opportunities and challenges this presents for the planning, installation, and operation of systems.
Modbus RTU – a successful model with an expiration date?
Anyone connecting a refrigeration system, air conditioning unit, or heat pump to a control system today is highly likely to encounter Modbus RTU via RS-485. Over the past few decades, the serial two-wire bus has effectively become the lowest common denominator in the RACHP industry: it is simple, robust, cost-effective, and supported by almost all component manufacturers—from compressors, fans, and electronic expansion valves to heat pump controllers and monitoring units.
This broad support is one of the keys to its success: plant engineers and designers usually only need to master one fieldbus system. From the manufacturer's point of view, a single bus interface is often sufficient to cover the majority of market requirements. Modbus RTU was therefore a pragmatic and economically clever solution for a long time.
But there is a downside to this success story. Modbus RTU was developed in the late 1970s, at a time when neither cybersecurity, cloud connectivity, nor detailed status monitoring played a major role. Important limitations are becoming increasingly apparent today: the data rate is limited, the number of participants per bus is restricted, broadcast or multicast mechanisms are lacking, and, above all, there are no integrated security mechanisms. Authentication, encryption, or secure firmware updates must be laboriously retrofitted in higher protocol layers, if at all.
At the same time, external pressure is growing. Digitalization, energy efficiency requirements, sustainability reporting, and new cybersecurity regulations call for a more modern communications infrastructure. At this point, at the latest, the question arises: Will Modbus RTU remain the dominant standard for a long time to come—or is there a need for an IP-enabled, more secure, and more powerful alternative?
Ethernet in a nutshell – and what Single Pair Ethernet makes of it
To put the discussion into context, it is worth taking a brief look at Ethernet. Ethernet is the dominant network technology in the IT world – in offices, data centers, and increasingly in industry. Technically speaking, it is a family of standards in which data is transmitted in packets via a physical medium – copper cable, fiber optics, radio. Above this physical layer run protocols such as IP, TCP, UDP, HTTP, MQTT, OPC UA, or Modbus TCP, which ensure that devices can find each other in the network, transmit data reliably or particularly quickly, and provide services such as web interfaces or cloud connections.
Classic Ethernet typically works with four twisted pairs of wires (e.g., in an RJ45 connector) and cable lengths of up to 100 m between two active network devices. This is ideal for server rooms and office floors, but often too complex and “heavyweight” for connecting field devices and sensors in the field.
This is exactly where Single Pair Ethernet (SPE) comes in. From a technical point of view, SPE is “normal Ethernet,” meaning it uses the same protocols as classic Ethernet—i.e., IP, TCP, MQTT, OPC UA, and the like. The key difference lies in the physical implementation: SPE requires only one twisted pair of wires. This reduces the amount of material required, enables smaller and more robust connectors, and allows for longer cable lengths. In the 10BASE-T1L variant, for example, ranges of up to 1,000 meters at 10 Mbit/s are specified. In addition, Power over Data Line (PoDL) can be used to supply power to end devices with up to approximately 50 watts via the same twisted pair.
For the user, this means that at the protocol level, SPE is full-fledged Ethernet, while at the cabling level, its design and handling are closer to what is familiar from RS-485 buses—namely, a simple two-wire cable with industry-standard connectors according to IEC 63171.
Why Single Pair Ethernet is exciting for refrigeration and air conditioning technology
The refrigeration, air conditioning, and heat pump industry is undergoing massive change. On the one hand, the number of networked components in systems is growing: more sensors, intelligent compressors, frequency-controlled pumps, networked heat pumps, distributed cooling centers, and integrated building automation systems. On the other hand, there are increasing demands for transparency and controllability – keywords being energy costs, decarbonization, and regulatory reporting.
Today, these requirements are often met with a mix of Modbus RTU, proprietary fieldbuses, remote I/O modules, and gateways to BACnet, Modbus TCP, or cloud connections. The result is an isolated communications landscape: Modbus RTU dominates below the IP level, while above it there is a variety of protocols and manufacturer specifications. Each translation stage (gateway) costs money, creates latency, and is a potential source of error – and often also a security risk.
The vision of an SPE-based approach is different: Ethernet communication should no longer end in the control cabinet, but extend all the way to the sensor or actuator. Compressors, fans, expansion valves, heat meters, room devices, or refrigerant pumps would be directly IP-enabled in the future and could be connected to a common Ethernet network via SPE. Gateways would not disappear, but there would be significantly fewer of them and they could focus on pure protocol integration (e.g., to a manufacturer's backend) instead of translating serial fieldbuses.
Advantages of Single Pair Ethernet compared to Modbus RTU
The most important difference from the user's point of view is end-to-end IP communication. While Modbus RTU is a fieldbus without IP functionality, SPE is based directly on Ethernet/IP. This enables clean end-to-end communication from the field device via switches, routers, and firewalls to the building management system or the cloud. Process data, diagnostics, historical trends, and firmware updates use the same communication principle as office PCs or servers. For operators of large plants, this means a standardization of the network world: IT and OT are growing together instead of being laboriously linked via gateways.
A second, very practical advantage lies in the data rate. Even at 10 Mbit/s—the slowest SPE variant—there is orders of magnitude more bandwidth available compared to typical Modbus speeds (in the kBaud range). This allows far more than just target and actual values to be exchanged. Detailed diagnostics, comprehensive event logs, and cloud analytics become possible without immediately “clogging up” the network. This is a crucial point, especially with regard to predictive maintenance and condition monitoring, for example in compressors or larger refrigeration systems.
SPE also draws directly on the established cybersecurity mechanisms of the IT world. Encrypted connections, certificate-based authentication, role and rights concepts, segmentation through VLANs and firewalls – all of this is already available in Ethernet and firmly anchored in standards, best practice guides, and products on the market. Modbus RTU, on the other hand, offers no protection whatsoever; any form of authentication or encryption must be laboriously superimposed – or is omitted altogether for cost reasons. In view of new legal requirements for cyber resilience, this is an increasingly difficult risk to justify.
Added to this is the aspect of power supply via the same cable. With PoDL, an SPE strand can not only transmit data, but also supply compatible field devices with power at the same time. For many components in refrigeration and air conditioning technology – such as sensors, room controllers, or communication modules – the specified power classes are completely sufficient. Separate low-voltage power supplies and additional supply cables are no longer necessary. Installation times and material costs are reduced, the space required in distribution boards is reduced, and the fire load from cables is reduced.
Last but not least, SPE also offers environmental benefits. Because only one pair of wires is required, copper and plastic consumption is significantly reduced compared to four-pair Ethernet cables. In conjunction with PoDL, additional power lines can often be dispensed with entirely. This enables considerable resource savings in large systems or series products—an argument that is becoming increasingly important in times of CSRD reporting requirements and sustainability goals.
Weaknesses and stumbling blocks from a practical perspective
As convincing as the technical advantages of SPE appear on paper, it is important to consider the practical challenges that a switchover entails.
The first and most obvious point is the cost at the device level. A simple RS-485 transceiver for Modbus RTU is very inexpensive and has minimal computing power and memory requirements. An SPE-capable device, on the other hand, requires a significantly more complex Ethernet PHY, often a more powerful microcontroller with IP stack, security libraries, and update logic. In the short term, this means higher unit costs, especially for simpler devices in high volumes. One thing is certain: as SPE components become more widespread, prices will fall. During the transition phase, however, cost pressure remains a serious objection, especially in highly price-sensitive markets.
Another aspect is installation practice. Refrigeration engineers are used to Modbus buses: two wires plus shielding, terminating resistors, addressing, possibly one or two repeaters – that's often all it takes. Ethernet, even in the form of SPE, brings other issues into play: IP addressing concepts, subnet planning, switch configuration, and possibly VLANs and firewalls. This is unfamiliar to many installers and commissioning engineers at first and can lead to errors and frustration if the introduction is not accompanied by appropriate training and support.
In addition, industry-specific profiles for SPE are only just emerging in the RACHP world. With Modbus RTU, de facto standard registers and data models have become established over the years, even if they are not always perfectly documented. For SPE-based communication, such consolidated profiles are still in their infancy. The cooperation between ASERCOM and the Single Pair Ethernet System Alliance has precisely the goal of developing clear recommendations at an early stage so that not every company defines its own incompatible Ethernet variant for refrigeration and air conditioning equipment. However, as long as this work is still in progress, there is a risk of IP-based fragmentation.
The migration issue should not be underestimated either. The installed base of Modbus devices is huge and will not disappear overnight. Realistically, Modbus RTU and SPE-based solutions will coexist for many years to come. The transition will require gateways, hybrid devices with both interfaces, and transition concepts. This will increase the complexity of the system, at least temporarily, before it can decrease in the long term. Anyone who wants to use SPE in new projects must therefore carefully plan how old and new systems will communicate with each other and what strategy should be pursued for service and spare parts management.
Finally, the question arises as to whether a full-fledged Ethernet stack is simply “oversized” for very simple applications. For local, non-networked temperature sensors or pure on/off signals, a simple serial solution may still be the most pragmatic option. It is quite conceivable that there will be coexistence in the long term: SPE/Ethernet where security, remote access, analytics, and interoperability are paramount, and lean serial solutions where simplicity and price dominate.
A joint industry approach instead of isolated solutions
In order for SPE to actually play a similar role to Modbus RTU in refrigeration, air conditioning, and heat pump technology, it is crucial that the industry acts in a coordinated manner and does not everyone goes their own way. This is precisely why ASERCOM and the Single Pair Ethernet System Alliance are working together in a joint working group. The aim is to avoid a proliferation of variants and to define common denominators at an early stage:
Conclusion: Evolution instead of revolution
Single Pair Ethernet is a highly interesting candidate for the role of a “Modbus successor” in refrigeration, air conditioning, and heat pump technology—but not as a direct drop-in replacement, rather as part of a gradual technological evolution. The technology offers many features that will become indispensable in the coming years: consistent IP communication, integrable cyber security, higher data rates, power supply via the data line, and better environmental performance indicators. At the same time, the hurdles in the form of higher hardware costs, more development and training effort, and a long migration phase are real and should not be underestimated.
From today's perspective, it is unlikely that Modbus RTU will disappear from systems in the short term. A more realistic scenario is a longer period in which both worlds exist in parallel: SPE for new, more networked and safety-critical applications, Modbus RTU in existing systems and simple tasks. The more successful the development of industry-standard profiles and architecture guidelines for SPE, the more attractive the technology will become for manufacturers, plant engineers, and operators.
Whether SPE will take over Modbus RTU's role as the dominant communication system in the RACHP industry in the long term therefore depends less on the physical transmission technology—which has largely been resolved—and more on standardization, interoperability, training, and economic conditions. ASERCOM and the Single Pair Ethernet System Alliance are currently working together on precisely these issues. Those who follow this development early on and plan initial pilot projects will gain an important head start in terms of experience for the communications infrastructure of the next decade.
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