Digital Network Reliability

As we commit our control system design to include digital networks rather than hardwired I/O, we want to be sure about reliability and know where redundant network devices are most needed, even if we specify hardened devices for many of the components. We worry that adding unnecessary network complexity and cost will give us after-sales support headaches. We'd like some seasoned advice.

—From December '11 Control Design

Answers

Reliability Important

With the digital network now transporting the information of many I/O points, the reliability of that network and its infrastructure (cable, switches, etc.) is that much more important than the individual I/O cable it replaces.

One way to increase reliability and network availability is to add redundancy to create a system that can tolerate one network failure. Depending on the network architecture, this can be achieved in different ways.

A line architecture can be upgraded to a ring by closing the loop between the last and first device in a line. If the data transmission is interrupted in one direction, it can then be rerouted automatically in the other direction. With a ring redundancy (Figure), a single fault can be tolerated without loss of communication and without adding additional network components besides the cable closing the loop.

But before going through the expense and sometimes increased complexity of adding network redundancy, some other aspects should be considered when choosing a digital network. One is network topology. Some networks require a line architecture, meaning that all devices on the network have to be in the same segment. A failure in such a network means loss of communication to all devices behind the point of failure. Other networks allow much more flexibility in network architecture such as line, star, tree, ring or any combination of those types. Using a mix of line and star architecture for network segmentation can allow for the remaining network segments to function during a failure in one segment without investing in redundant network components.

A final consideration is the current separation—and thus quasi-redundancy—of regular I/O and safety I/O via different hardwired signals or networks, leading once again to increased cost and system complexity. Technology like openSafety allows data of safety devices to be transported over the same digital network as regular I/O data. Integrating safety into digital networks will therefore reduce the necessary network and hardwired infrastructure, and allows the same network flexibility in terms of architecture in general and redundancy in particular to be used for all I/O, both regular and safe.

Robert Muehlfellner,
Director of automation technology,
B&R Industrial Automation, www.br-automation.com

Points to Consider

Because networked I/O solutions will be able to provide a plethora of additional diagnostics data, one might conclude that they have to be more complex. Unfortunately, in many instances, that is the case. If you intend on benefiting from the diagnostic advantages and the added flexibility these solutions offer, and possibly even put your safety devices on the same network, you have to be willing to tackle a certain learning curve. Here are a few guiding questions (and answers) that you might want to use when picking a digital I/O technology:

1. Is it possible to work with the network without using a specialized software tool? Ideally, starting up and maintaining the network can be completed without additional PC configuration software.
2. How much downtime is necessary when more I/O must be added to the network? Adding field I/O to the network should be accomplished in minutes.
3. Can the same networking solution be used with PLCs from different suppliers? You never know what happens at the PLC market. Any networking solution that can only work with a small number of PLCs is problematic.
4. Is the technology deterministic and fast enough for the application? A nondeterministic solution is a nonstarter. I/O update times should be twice as fast as PLC scan times. Less than 10 ms is probably a good number.
5. Does the technology support machine safety operations? Even if this is not what you have in mind right now, using a technology that does not support safety will be a problem in the future. Networking safety results in a reduction of wiring complexity as high a 90%. Any selected technology should at least be ready.
6. Can a field module be exchanged without additional tools? You hope that the network you select is reliable. This means that I/O module exchanges are infrequent and knowledge about the solution is not well retained by maintenance. Module exchange must, therefore, be as simple as "removing the old, inserting the new."

I also urge you to take advantage of the expertise your chosen supplier brings to the table. Get them involved, and describe the application as best as possible. Your supplier can then review your layouts and designs, thus reducing cost and making the digital network run at peak performance.

Helge Hornis
Manager, Intelligent Systems Group
Pepperl+Fuchs, www.pepperl-fuchs.us

Congratulations Are Due

Congratulations on embracing a fieldbus network instead of hardwired I/O. I believe everyone in the fieldbus communities will state unanimously that with distributed I/O you will capitalize on superior diagnostics, simpler wiring, improved uptime and overall cost reductions compared with your current hardwired implementation.

Since your concern seems to be with the communication network itself, the need for controller or even I/O device redundancy does not seem to be your need or concern, but instead what the industry calls "added availability" or, more informally, "network redundancy." Basically, this is a scheme that enables the system to withstand faults in the communication network, such as a cable shear, unplugged cable, or dead module.