Ethernet was first standardized in 1983 by the Institute of Electrical and Electronic Engineers (IEEE). Originally specified for a maximum of 2.94Mbit/sec, it has been revised many times to support higher bit rates, a greater number of nodes, more operating configurations and longer distances. Today Ethernet is the dominant architecture for local area networks (LANs). Initially developed for computers and data processing, it has expanded into industrial automation, process monitoring, and instrumentation applications.
Ethernet Architecture
Ethernet is a method of wired (copper or fiber) communication that offers much higher speed than USB, RS-232 or RS-485. It is a packet-switched network with multiple access points (shared media) and no central control. Systems using Ethernet divide a stream of data into frames. Each frame contains source and destination addresses, data packet and error-checking (so that damaged frames can be detected and resent). An Ethernet network is usually configured in a star topology with every device connected to a switch or router. In this diagram of a simple network, five measuring instruments and one computer are connected to a 12 port Ethernet switch.
The latest versions of Ethernet support bit rates up to 400Gbit/sec. Since most instrumentation applications do not generate huge amounts of data, they typically use 10BASE-T, 100BASE-TX (also known as Fast Ethernet) or 1000BASE-T (Gigabit Ethernet). These have a maximum speed of 10Mbit, 100Mbit or 1Gbit per second, respectively. The T suffix refers to transmission over twisted pair. Almost all Ethernet devices operate in full duplex for simultaneous transmit and receive. Full duplex Fast Ethernet uses 2 of the 4 twisted pairs in the Ethernet cable. Gigabit Ethernet uses all 4 pairs. The standard cable has an 8-pin RJ45 connector at each end. Maximum length is 100 meters (328 feet). Unshielded twisted pair (UTP) CAT 5e cable is adequate for all three bit rates, but CAT6 (or 6a), with lower crosstalk, is recommended for 1000BASE-T.
This photo shows an RJ45 connector for copper Ethernet and a shielded version with metal shell. On the right is a pair of ST connectors (with plastic protective caps) for full duplex fiber Ethernet. Fiber versions of Ethernet can support much longer distances. They also provide electrical isolation and are immune to electromagnetic interference. 100BASE-SX defines 100Mbit speeds over 850nm fiber. 100BASE-FX offers this speed over 1300nm fiber. Comparable standards at 1Gbit are 1000BASE-SX and 1000BASE-LX. Common connector types are ST, SC and LC. Most gigabit fiber is terminated at an SFP (Small-Format Pluggable) module in the switch or device.
Ethernet switches route messages to the correct device. An unmanaged switch may be sufficient for a small network. It simply negotiates with each device for the appropriate speed and duplex mode and requires no user setup. A managed switch offers more configuration choices, security features and network maintenance tools. It is recommended for larger networks. A router combines managed switch capabilities with internet access for all connected devices. It also is used to connect a LAN (local area network) to a WAN (wide area network).
Most Ethernet devices have a power connection separate from the data connection. To simplify installation and operation, small devices, like remote sensors and indicators, may utilize Power over Ethernet (PoE) from a managed switch to obtain up to 71W of low voltage DC through the same cable used for data transmission.
Ethernet Protocol
The protocol is the common language that each device on the network uses for communication. A popular Ethernet protocol is Modbus TCP, which exchanges Modbus messages over TCP/IP networks. The Internet Protocol (IP) is a set of requirements for addressing and routing data packets on the Internet. Once the data arrives at its destination, it is handled by the Transmission Control Protocol. Many internet applications, including the World Wide Web, email, and File Transfer Protocol, use TCP extensively. It is optimized for accurate data delivery rather than precisely timed delivery.
Measurement Applications
Industrial sites today commonly use Ethernet to connect PCs, PLCs, remote I/O, HMIs, smart instruments, motor drives and other automation components.
Here is a sample network for the test area in a facility. In this star configuration, each instrument has a full duplex 100BASE-T connection to the managed switch. Remote sensors connect to, and are powered by, a separate switch close to the sensors. Interface to the corporate part of the network is via Gbit fiber Ethernet. Controllers, recorders, power monitors, panel meters, and data acquisition systems are typical instruments that offer Ethernet communication.
Ethernet networks in harsh environments may require ruggedized switches with extended temperature range and enhanced transient protection. Connectors sealed against dirt and moisture are often used instead of standard connectors. An overall foil shield (FTP) is sometimes specified on cabling for improved noise immunity. Ethernet cables with individually shielded twisted pairs are also available.
Some instruments allow a number of like devices to share a single Ethernet connection. In this diagram, the first Laurel panel meter has the Ethernet interface and acts as an RS-485 to Ethernet server for the other Laurel daisy-chained meters.
Manufacturers of Ethernet-capable instrumentation usually provide utility software to configure the unit for network operation. This typically includes setting the network address, entering other communication parameters, node discovery, naming, selecting time zone, and entering email addresses. This setup can also be done from the front panel in some products.
Modbus TCP/IP is usually adequate for monitoring a process or extracting data stored in an instrument. Response time can be 100ms or more. Applications that require precise real-time control and tight timing between devices often use a specialized industrial protocol, such as PROFINET RT, to provide lower latency than TCP/IP.
Summary
Ethernet is a simple and cost-effective way to network instrumentation and share process or test data within a facility. IP capability provides access across physical and geographic boundaries. Compared to Wi-Fi, Ethernet offers higher transmission reliability, better noise immunity, faster speed, and more consistent latencies. Linking Ethernet devices in production, test and engineering with the corporate enterprise facilitates data sharing and quicker decision-making. Web access to Ethernet instrumentation allows remote monitoring and management. As purchase, installation and maintenance costs continue to decline, Ethernet will become a viable option for more test and measurement products.