Manufacturing AUTOMATION

Features Networks Technology
The future is now


May 14, 2013
By Ivan Romanow

Topics

The convergence of industry and technology, minds and machines, hardware and software, intelligence and connectivity can provide multitudes of efficiencies from better scheduling and fuel savings to better maintenance. But all that must start with getting useable data from the plant floor.

Selecting industrial networking protocols helps improve production efficiency and quality with enterprise connectivity, although Ethernet of the industrial kind requires specialty knowledge and practices beyond Ethernet for home and office. When installing or operating an industrial Ethernet network, there are some key essentials to consider, including cabling, signal quality, ground loops, switches and traffic.

To help manufacturers accommodate evolving networking requirements, such as decentralization of control, integrated diagnostics and simplified maintenance, network protocols integrate with industrial equipment and control systems to communicate crucial status updates and production data. This results in a powerful industrial tool to streamline manufacturing production with reliable, enterprise-wide connectivity, providing the highest level of visibility, control and flexibility achieving increased productivity and reduced operating costs.

With the migration away from point-to-point connection, advanced networking architectures ensure connectivity, collaboration and integration from the device level to enterprise business systems. By maximizing production control, enterprise connectivity can improve product quality, customer satisfaction and company profitability. When choosing a networking solution, users must understand the individual communication requirements as well as any environmental challenges present in each application. Evaluating the performance capabilities, features and characteristics of industrial protocols can assist manufacturers in selecting the ideal networking solution for critical communication needs.

Which cable is the right cable?
The Ethernet physical layer was developed with the primary purpose of conveying large amounts of information. Applied first to office-level networks, where multiple clients use the network to share information, Ethernet has expanded beyond traditional usage to the plant floor, especially with the advent of industrial Ethernet protocols. Ethernet communications can be used for industrial data collection, transmission and monitoring.

Industrial environments are subject to temperature ranges, dust, humidity and a host of other factors not normally found in a home or office. What’s the right choice of cable? In an office, commercially-rated cable like Category 5 is good for up to 10 MB and Category 5e is good for up to 100 MB. The ANSI/TIA-1005 standard states that Category 6 or better cabling should be used for hosts or devices that are exposed to an industrial environment.

Category 6 cable is good for up to 1 GB at 100 meters and 10 GB at 55 meters. Category 6a cable can do up to 10 GB at 100 meters. Category 6 cable is generally less susceptible to cross talk and external EMI noise than Category 5 and 5e cables.
Industrial Ethernet cables are designed to be less susceptible to physical deterioration in the harsher industrial environments. When installing Category 6 cable, ensure that the RJ45 ends and jacks are also rated for Category 6. For the best results, use premade patch cables for short runs, with factory installed connectors. For long runs, install jacks.

Cables, shielding, ground loops
Some applications require shielding, but improperly installed shielded cable can create more problems than it solves. Shielded Ethernet cable may perform better in high EMI environments if run outside of conduit.

Proper grounding is critical with shielded cable. One ground reference is essential. Multiple ground connections can cause ground loops, where the difference in voltage potential at the ground connections can induce noise on the cable.
A ground loop can wreak havoc on your network. To get this right, use a grounded RJ45 connector on only one end of the cable. On the other end use a nonconductive RJ45 connector to eliminate the possibility of ground loops. If the Ethernet cable crosses power lines, always cross at right angles.

Separate parallel Ethernet and power cables by at least eight to 12 inches or more for higher voltages and longer parallel runs. If the Ethernet cable is in a metal pathway or conduit, each section of the pathway or conduit must be bonded to the adjacent section for electrical continuity along its path.

In general, route Ethernet cables away from equipment that generates EMI, such as motors, motor control equipment, lighting and power conductors. Within panels, separate Ethernet cables from conductors by at least two inches. When routing away from EMI sources within a panel, follow the recommended cable bend radius.

Fiber optics
Fiber optic technology has many benefits for industrial networks, including high levels of electrical insulation and isolation, easy installation and survivability in hostile environments. Upgrading can bring these and more, such as greater security, robustness and signal integrity.

Designers from many industries turn to fiber optic data links as an alternative to copper media. Fiber optic solutions result in reliable data links that are capable of communicating over distances—ranging from inches to kilometres—and are more immune to noise. Although both copper and fiber are used as a transmission medium, fiber optic solutions offer some clear benefits for the system designer. Industrial Fast Ethernet working hard clad silica (HCS), for longer data links, has numerous advantages over copper solutions. While copper-based communication links are susceptible to electromagnetic (EM) fields and emit EM noise, which may interfere with other instrumentation, fiber optic links are immune to EM fields and do not generate any electromagnetic interference (EMI).

Other advantages of choosing fiber over copper include: low weight, complete galvanic separation between link partners, easy field termination and maintenance, easier installation due to short bending radius and less susceptibility to performance changes caused by temperature extremes and humidity. Fiber optic solutions are also well suited for noisy, industrial environments that have motors and high-voltage.
At the higher networking level, industrial Ethernet connects engineering and management workstations to industrial Ethernet hubs for data sharing and control across the enterprise. The value proposition is significant. Fiber solutions are available with various data rates, like 1Gb and 10Gb, along with achieving distances of 2 km for multinode fiber and greater with single mode fiber with connectors that serve industrial communications and factory automation applications.
Best of all, fiber has been successfully applied for more than 30 years and is widely used in enterprise and industrial applications.

Switches and hubs
To put it simply, never use a hub in an industrial Ethernet environment. Hubs are nothing more than multiport repeaters. Eliminating the use of hubs leaves the choice between managed and non-managed (or unmanaged) switches. While managed switches are generally preferable, they are also more expensive than non-managed switches.

Every device on a network has a unique identifier, referred to as a media access control (MAC) address. This is the key to the much more discriminating behaviour of a switch compared to a hub. When a switch first powers up, it initially behaves like a hub broadcasting all traffic everywhere. As devices pass information between ports on a switch, it watches this traffic, figures out which MAC address is associated with which port and places this information in a MAC address table. Once it figures out the MAC address of a device connected to a particular port, it will watch for information intended for that MAC address and transmit such information only to the port associated with that address.

An industrial Ethernet network carries three types of traffic. Unicast traffic routes from one point to another point. Multicast traffic routes from one point to many points. Broadcast traffic routes from one point to all points. Once a switch has built its MAC address table, managed and unmanaged switches treat unicast and broadcast traffic identically.

Generally, keep broadcast traffic under 100 broadcasts per second, at a bandwidth of 100 Mb. A little bit of broadcasting is an integral part of any network. An example of devices that may initiate broadcasts is a print server, announcing itself periodically to the network.

Snooping
One of the primary differences between managed and unmanaged switches is how they treat multicast traffic. Multicast traffic typically comes from smart devices on plant floor process networks, in a connection-oriented producer-/consumer-based technology. In this context a connection is simply a relationship between two or more nodes across a network. A device needs to be a member of a multicast group to receive group data. All members of the group receive data. You do not need to be a member of a group to send data to the group. The main problem with multicast traffic in a producer/consumer model is that traffic grows exponentially with the number of hosts. This is where the managed switch comes in. A managed switch has the ability to turn on Internet Group Management Protocol (IGMP).

Here’s how snooping works. When enabled, IGMP snooping sends out broadcast traffic to determine the members of any multicast groups. This information, combined with the MAC address table, allows a managed switch to route multicast traffic only to those ports associated with members of a multicast group. A non-managed switch treats multicast data the same as broadcast data and sends it everywhere. If the network uses producer/consumer technology or has multicast traffic, a managed switch is a must and worth the price premium.

Troubleshooting
There are other reasons to consider a managed switch. This class of switches usually provides error logs, control of individual port speeds, duplex settings and the ability to mirror ports. These extra capabilities allow more precise control of network behaviour and can be an invaluable help troubleshooting issues that will certainly occur on the network at some point. When network performance issues occur, the first suspect often is the switch, though the switch rarely is the core of most network performance problems. Switches tend to be the lowest latency points in a system, typically operating 10 to 50 times faster than all other network components.

While there is excellent software to help troubleshoot network performance issues, most of it can only see broadcast and multicast traffic. That’s fair enough, because many performance issues are caused by unrestrained multicast traffic or excessive broadcast traffic. If you need to examine unicast traffic for any reason, port mirroring is the only way.

It is OK to use a non-managed switch if there is no multicast traffic on the network. On very small, simple networks with a few devices, many people use non-managed switches. Sometimes they take half-steps and combine the two, having a few remote devices on a non managed switch, which then feeds into a managed switch. As a general practice for networks of more than a few nodes, if cost is not a primary concern, go with a managed switch, which is often a much better choice in hindsight.

Ivan Romanow, CET, is director of sales and marketing with Gescan Ontario, a division of Sonepar Canada. For more information, visit www.gescanontario.com.

This article originally appeared in the May 2013 issue of Manufacturing AUTOMATION.