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Use Industrial Ethernet as a Rugged and Deterministic Alternative to Standard Ethernet

By Bill Giovino

Contributed By Digi-Key's North American Editors

Ethernet is the most widely used wired networking standard for local area networks (LANs) and wide area networks (WANs). While standard Ethernet communication often seems instantaneous, in reality, there is no guarantee data packets sent across the network will arrive at their destination port within a specified fixed time frame.

This non-determinism can be a problem in industrial applications where a delayed message can cause a production line shutdown or injure personnel. To provide this determinism, a modified “industrial” version of Ethernet can be used.

This article discusses the key elements of industrial Ethernet and looks at two Ethernet switches from Delta Electronics that can form the foundation of a deterministic industrial Ethernet network.

The need for determinism in Ethernet

Ethernet is a physical networking standard that defines the physical cables and connectors used to transmit data packets across a network. Standard Ethernet uses the TCP/IP suite of protocols to define how data is exchanged between devices connected to the network. However, even in a local network, TCP/IP is non-deterministic and can also introduce delays as high as hundreds of milliseconds. This is unacceptable for industrial local networks connecting industrial automation systems. These systems often depend upon packets, or messages, arriving within a defined time window and demand response times as fast as 10 milliseconds (ms), or in some extreme cases, faster than 1 ms.

Profinet is another popular Ethernet frame format (also called a communications protocol) that is used in industrial networks. Profinet was developed as a simpler alternative to full TCP/IP. A real-time version of it, Profinet RT, prioritizes data so that it can deliver high priority process control messages under tighter time constraints. Less time-dependent status messages would get lower priority. However, Profibus RT is still not deterministic. To provide determinism, an isochronous version of Profinet RT was also developed (Profinet IRT).

Profinet IRT achieves determinism by superimposing scheduled time slots on the network bandwidth: IRT data uses one slot and RT and IP data uses the other slot. In this way, devices on the IRT slot can be clocked to receive data deterministically.

It is important to note that if an industrial Ethernet network is based on a deterministic communications protocol such as Profinet IRT, then all devices connected to the network must also be Profinet IRT compatible. Consequently, typical TCP/IP devices such as IP cameras and hardware firewalls will not function on the network. This provides a layer of security by preventing personnel from operating unauthorized devices on the network.

Move from Ethernet hubs to ruggedized switches

An Ethernet hub acts as a repeater, making it inherently non-deterministic. Data packets received from a single port are forwarded to all other ports on the hub’s network. So, if 25 ports are connected to a hub, and if port 3 sends data packets intended for port 22, the hub will send the data packets not just to port 22 but to all ports. As all ports connected to the hub share the same bandwidth, as more ports are added to the hub they compete for that bandwidth. This increases the likelihood of collisions, delaying packets and making determinism impossible to achieve. In addition, because of this setup, a hub can only send one message at a time. If another port needs to send data, it must wait until the network is clear. This is why Ethernet hubs are inherently non-deterministic.

An Ethernet network switch, on the other hand, is an intelligent device. It stores the unique media access control (MAC) address of all devices plugged into the switch in an internal MAC address table, usually in high-speed RAM. Because the switch knows the MAC address of every device connected to it, it can forward data packets only to the port for which the data is intended. So, if port 3 sends data packets to port 22, the switch only sends the data packets to port 22.

A switch can manage multiple simultaneous data communications. As such, port 3 can send data to port 22 at the same time that port 4 is sending data packets to port 7. This eliminates the collisions inherent with Ethernet hubs, while also increasing the speed, efficiency, and throughput of the entire network. Because the only delay is the known switching speed of the switch (as specified in the switch’s datasheet), network switches are required when building a deterministic Ethernet network.

A network switch also brings an added layer of security. Since a port will only receive the data packets intended for it, the network cannot be monitored by a device plugged into one port.

Switches for industrial Ethernet are also more rugged compared to commercial Ethernet switches. While commercial switches are appropriate for light duty home or office use, industrial Ethernet switches are able to withstand harsh factory environments and can stand up to more intense vibration, shock, and temperature extremes. In addition, industrial Ethernet switches are designed to be safe for use in the presence of explosive gases and are resistant to corrosive atmospheres.

Ethernet on DIN rails

Industrial Ethernet switches are epitomized by Delta Electronics’ 5-port, DIN rail mount DVS-005W01 (Figure 1).

Image of Delta Electronics DVS-005W01 industrial Ethernet switchFigure 1: The Delta Electronics DVS-005W01 industrial Ethernet switch is a rugged switch that can withstand harsh industrial environments. It supports five Ethernet ports and is IP40 rated. (Image source: Delta Electronics)

Like any Ethernet switch, the DVS-005W01 only transmits data packets to the receiving ports. It supports 100Base-T transmission speeds of up to 100 megabits per second (Mbits/s). The MAC address table supports 1024 entries and a 512 Kbit packet buffer memory is available for storing forwarded packets. In addition, while commercial switches and hubs use a single power supply, the DVS-005W01 has two 12 to 48 volt redundant power inputs; so if one power supply goes down, a second backup power supply can be easily switched on without any additional external hardware.

The DVS-005W01 supports an internal switching speed of 1 gigabit per second (Gbps), which is the internal speed of the network switch when processing incoming and outgoing data packets. This is an important specification for deterministic behavior as the faster the switching speed, the shorter the delay.

For more complex industrial networks, Delta Electronics also supplies the DVS-016W01 Ethernet switch with 16 ports (Figure 2).

Image of Delta Electronics DVS-016W01 industrial Ethernet switchFigure 2: The Delta Electronics DVS-016W01 supports 16 Ethernet ports and mounts on a DIN rail or wall mount. (Image source: Delta Electronics)

The DVS-016W01 has a MAC address table that supports 8192 entries and has a packet buffer memory of 1 Mbits. It supports an internal switching speed of 3.2 Gbps.

Both the Delta Electronics’ 5-port and 16-port switches are unmanaged switches, which means they are basically plug and play. Whenever a network device is plugged into one of the ports, the switch automatically detects the MAC address of the device and stores that address in its internal MAC address table. Auto detection of MAC addresses is also performed whenever the switch is power cycled. Managed switches have more per-port traffic control features that need to be configured upon setup.

To complement the switches, industrial cables for Ethernet are available with IP67 or IP68 ratings. Such cables for Ethernet are heavy duty and can withstand temperature extremes of hot and cold, corrosive solvents, immersion in water, and other abuse. Cables for industrial Ethernet are also more forgiving when it comes to the bending, twisting, stretching, and impact forces that cables in factories can encounter.

Conclusion

Industrial automation applications have requirements for communications latency, determinism, and hardware ruggedness that go far beyond the capabilities of standard Ethernet. However, as shown, designers and line operations engineers can provide industrial strength Ethernet using deterministic communications protocols and off-the-shelf switches and cabling.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Bill Giovino

Bill Giovino is an Electronics Engineer with a BSEE from Syracuse University, and is one of the few people to successfully jump from design engineer, to field applications engineer, to technology marketing.

For over 25 years Bill has enjoyed promoting new technologies in front of technical and non-technical audiences alike for many companies including STMicroelectronics, Intel, and Maxim Integrated. While at STMicroelectronics, Bill helped spearhead the company’s early successes in the microcontroller industry. At Infineon Bill orchestrated the company’s first microcontroller design wins in U.S. automotive. As a marketing consultant for his company CPU Technologies, Bill has helped many companies turn underperforming products into success stories.

Bill was an early adopter of the Internet of Things, including putting the first full TCP/IP stack on a microcontroller. Bill is devoted to the message of “Sales Through Education” and the increasing importance of clear, well written communications in promoting products online. He is moderator of the popular LinkedIn Semiconductor Sales & Marketing Group and speaks B2E fluently.

About this publisher

Digi-Key's North American Editors