Completing the industrial Ethernet connection

Specifications, industrial environment and network structure are factors to consider when selecting devices for an industrial Ethernet infrastructure. However, getting data from device to device is another important factor. Ethernet wiring types, categories, cabling and connection options are also important components within the industrial Ethernet infrastructure.

Cable and connector issues

There are several naming conventions used in Ethernet. A "T" or "TX" refers to twisted pair cabling. "FX" refers to glass fiber optic cabling. For example, a 100-Base-FX specification means 100 Mbps (Fast Ethernet) over fiber cable. A "10-Base-T" means 10 Mbps over twisted pair copper cable. Copper cable is used for convenience; fiber optic cable is used for distance and noise immunity. The number of available cabling options is growing. Each has its benefits and drawbacks (See "Alternatives for 10/100 Mbps Ethernet cable").

Copper cable

Copper cable length is limited to 100 meters between any two devices regardless of data rates. The higher the copper cable category rating number is, the higher the frequency it is able to handle. Unshielded cables are popular in the US; shielded cables are popular in Europe. When using 100 Mbps Ethernet in areas of high electrical noise, shielded cables can provide added noise immunity.

For industrial applications, Category 5 (CAT 5) is the most widely used. CAT 5 cables have two twisted pairs, are low cost and handle 10/100 Mbps data rates. CAT 5E (E = enhanced) cable contains four twisted pairs and can handle data rates up to 1 Gbps, but can be more expensive than CAT 5, depending on the vendor. CAT 6 cables also handle 1 Gbps, but have greater electrical noise rejection capability than CAT 5E, and are more expensive than CAT 5E. They can also handle 10 Gbps signals. Though the CAT 5E and CAT 6 cables are more expensive, costs have been dropping and many companies are installing it now to avoid rewiring in the future.

Virtually all industrial devices have 10/100 Mbps connections. The automation industry is in transition from 10 Mbps to 100 Mbps-based systems. However, due to 10-times greater noise sensitivity and increased cost factors, gigabit Ethernet installations are rarely used in industrial automation systems. When they are, they are used to create a main Ethernet "backbone."

When selecting Ethernet cables for applications near cutting fluids, special cable coatings are required. Use cables with thicker outer coverings between panels. Thinner-walled cables are easier to bend for in-cabinet wiring.

Fig 1. When connecting Ethernet devices on machines, or in wet or washdown applications, consider using IP65, IP66 or IP67-rated connectors. The first numeral in the rating indicates protection from ingress of solid foreign objects. In this case, the "6" means that the connector is dust tight. The second numeral indicates protection from ingress of liquids. The "5" means jetting water, any direction; "6" means powerful jetting water, any direction; and "7" means temporary immersion in water.

The RJ45 connector is adequate for most industrial applications. But in applications near moving machinery, RJ45 connectors with locking hoods should be used to provide added strain relief and shock/vibration immunity. There are also many control panel bulkhead connection options available for connecting programming panel and device cables to enclosures without needing to open the cabinet. Water-tight IP65/67 connectors are available for connecting Ethernet devices in modular machine mount or wash down applications (Fig. 1).

Fiber optic cable

Although copper cables are the most popular, fiber optic cables are necessary for longer distance runs and noise immunity in high (electrical) noise areas. Multimode glass fiber is the most widely used. At 100 Mbps, it allows distances of 2,000 meters or more between Ethernet devices. The new generations of terminations have reduced the complexity of terminating multimode glass fiber cables experienced in the past.

Older multimode installations may use ST style of connectors that were popular with 10 Mbps Ethernet systems. Newer systems use SC style connectors recommend for 100 Mbps installations.

When upgrading existing fiber optic systems with new equipment, also check that the wavelength of the light sources are compatible. Today's 100-FX systems are based on 1,300-nanometer light. Older systems may be based on 850-nanometer light. When adding new switches to existing installations, check switch data sheets for the frequencies/wavelengths that the fiber interfaces support.

Unmanaged switches may have one or two fiber optic ports. These are used to connect the switch to the main network (backbone) or main control panel. A single port is used when the control panel is physically isolated and is connected through one long fiber cable. Dual fiber port connections are typically used to daisy-chain multiple switches. This saves fiber cable costs by eliminating individual long "home run" cables back from each switch to a central control cabinet switch.

For very long distances, single-mode telecommunications-grade fiber cables allow distances greater than 15 kilometers between Ethernet devices. Single-mode cables/connectors can be 4 times more expensive than multimode cabling, and require precise terminations by well-trained personnel. Using pre-terminated cables or professional installers can prevent reliability problems or system startup delays.

Plastic-clad glass (HCS) and new all-plastic POF fiber optic cables have recently emerged with 100 Mbps data-rate capability. They have shorter distance limits of 50–200 meters, but they are the most easily terminated. The POF connectors can be terminated reliably in 2–3 minutes without special tools. This makes them useful when routing within a cabinet around drives or starters; near welders, robots or large motors; or as a faster means of terminating custom cable lengths (vs. terminating RJ45 connectors). Plastic cables typically use SMA style connectors.

Fig 2. Copper, glass fiber and plastic fiber cable types can be mixed in most modular industrial Ethernet switches.

Control device connections

Fig 3. Media converters allow copper cable connections to be connected to any of the fiber optic cable types for long distance or EMI noise immunity requirements. Serial converters allow control devices that communicate using one of the serial protocols to be connected to the industrial Ethernet infrastructure. I/O systems allow real-time communication between PC/PLCs and sensor/actuator devices. Gateways translate the TCP/IP protocol to the fieldbus with which you wish to communicate.

Media converters

Virtually all industrial control devices today connect to Ethernet using copper cables with RJ45 connectors. However, media converters are used when fiber cable is needed to connect a device to the network. Media converters convert copper cable connections to one of the various fiber optic cable types. They are also useful for connecting devices to 850-nanometer legacy fiber optic systems or to new 1,300-nanometer fiber systems. Because the fiber optic ports on unmanaged switches are typically used to connect to other switches, media converters at the switch may be required when a third (or more) fiber connection is needed to go to an isolated device or cabinet. Media converters are available that support all glass and plastic fiber types.

Serial servers

Serial communications or device servers are used to connect existing RS-232 or RS-485 devices to Ethernet, replacing long serial cable runs. These servers insert serial transmissions inside the Ethernet packet and transmit them to either another serial server or a PC with software that strips out the information at the receiving end. These are intelligent devices that require IP addresses. They are configured through their web pages or serial ports.

I/O systems

Connecting discrete, analog, encoder and other control device types is accomplished via I/O stations. Ethernet I/O is available in form factors optimized for different applications. Modular Ethernet I/O is used in main control panels that have a greater quantity or variety of signal types to interface through. Block style I/O provides a fixed configuration of I/O in smaller sizes and at lower costs. These are optimized for small distributed I/O drops for multiple sections of a machine, or for monitoring systems. IP65/67 machine/process-mount I/O is also starting to emerge.

Other than the typical I/O system feature differences among vendors, there are several Ethernet-specific issues to consider. The main issue is deciding which real-time control protocol will be used. This is usually specified by the PLC or PC-based control system in use. Modbus/TCP, EtherNet/IP, Profinet and Foundation Fieldbus HSE for process control applications are the current major offerings. Some vendors standardize on one Ethernet protocol; others offer several for the same I/O form factors.

For monitoring applications, some I/O systems allow data and diagnostics to be viewed with Web browsers, reducing the need for higher-cost HMI/SCADA packages. There is a wide variability of diagnostics supported by available I/O systems. Most have Web-based diagnostics, link LEDs that show status (electrical and Ethernet connections OK or not OK), communication activity indication and any LEDs required by the protocol specification. Some may have numerical displays that can provide added fault-code diagnostic information without software. For example, a display could indicate that the device is waiting to be assigned an IP address.

A major option to check is fault response status. This feature determines whether the I/O system is to turn outputs off, leave in last operating state or go to a predefined state in the event communication is interrupted. In fieldbus systems, if a cable is cut, the physical connection is lost. The I/O station detects this and the outputs go into a predefined fault state (usually off).

With Ethernet, the connection between a PC/PLC and an I/O station might pass through many Ethernet switches. If a cable is cut between the middle two switches, it doesn't affect the immediate connection between the I/O and the immediate switch to which it is connected. Without additional attention, the outputs would be held in the last state while the I/O waits for output update information that will never come.

Some protocols include a built-in timer. If an output command or a heartbeat signal is not received within a certain delay after the last update, the I/O assumes the communication is lost and goes into a fault state. Some protocols do not specify this function and it is up to the vendors to add the timer in the I/O device as a product feature. If one is using a new Ethernet I/O control protocol, ask the vendor how these types of faults are handled.

Gateways

The major fieldbus protocols offer a gateway to the Ethernet implementation of their protocol. This allows existing fieldbus installations to be upgraded with Ethernet capability.

Conclusion

The new generations of Ethernet infrastructure components allow the cost effective implementation of distributed wiring approaches. Modular switches allow tailoring of port capacity and cable types while incorporating standards based redundancy and advanced message filtering functions. Device servers, media converters and I/O systems allow the types of control devices to be tied into a system in phases. Industrial infrastructure devices with easy to understand PLC-like Web pages and network management software allow networks that can be installed and maintained by plant floor personnel, yet are compatible with the IT world's standards and support tools. Flexibility, performance and lower lifecycle costs are key elements for enterprise-wide industrial Ethernet integration and growth.

The Bottom Line...

• Copper cable is used for convenience; fiber optic cable is used for distance and noise immunity.
• When upgrading existing fiber optic systems with new equipment, check that the frequency of light is compatible.
• Some I/O systems allow data and diagnostics to be viewed with Web browsers, reducing the need for higher-cost HMI/SCADA packages.
• Larry Komarek can be reached at (717) 944-1300, ext. 3625 or at lkomarek@phoenixcon.com.