Bill's Blog on Automation

Does Your Fiber Data Freeze?

Occasionally during winter, fiber optic cable will mysteriously fail then later return to proper operation. The failure could degrade speed and quality of traffic, or completely interrupt data communications. If this has happened to you, it could be the result of water freezing within the cable conduit.

Frozen water in ducts or conduits has always been a potential communications issue, but why should sealed and jacketed fiber optic cable be susceptible? The risk is that the force of water crystallization can bend fiber optic strands to the extent that cable performance is impaired. Signal quality usually returns with the thaw, but in rare cases cable can be ruined.

When water penetrates duct work (by infiltration or condensation) it flows down hill, resulting in substantial water in the lowest part of the duct. If a below-ground installation is under the frost line, there is no problem. But for shallow burials or exposed runs, ice can form in the duct.

Most freezing occurs near entrance points to buildings or other above-ground structures. It could also occur where soil erosion has rendered a previously acceptable burial depth too shallow — or simply due to improper installation.

One solution is rerouting the cable, but in many existing burials this could be very expensive. An alternative is to inject some substance (similar to the anti-freeze in your car radiator) to force water out of the portion the duct that is subject to freezing. Drilling weep holes is not recommended because it may not eliminate all instances of trapped water and clogged weep holes also will become a maintenance issue.

Your cable contractor should recommend the best solution for you.

Industrial Ethernet a Hit at SPS 2006

During November 28–30, over 43,000 visitors (a record) attended SPS/IPC/DRIVES 2006 International Exhibition & Conference in Nuremberg, Germany. Europe's premier tradeshow for electric automation systems and components included 1,203 exhibitors from 31 countries and occupied some 77,500 sqm of floor space (also a record). Special venues were dedicated for Electric Drives and Motion Control, Mechanical Systems and Periphery, Control Technology and Sensors, Software and Sensors, and Control Technology and Interface Technology. Key exhibition topics were Ethernet in Automation, Motion Control, and Safety and Security in Automation. Several conferences addressed the role of Industrial Ethernet in automation. Specialized Ethernet information was also available from various organizations such as ODVA, PI (PROFIBUS and PROFINET International) and the EtherCAT Technology Group.

Contemporary Controls and its subsidiaries were represented by George Thomas and Bennet Levine from the USA, Jan Thriene and Joerg Wehnert from Germany, Peter Jefferson from the UK, and Basile Waite from China. The company booth featured a very popular race car game played over an Industrial Ethernet redundant ring.

What Is MTBF and What Good Is It?

From time to time, a caller asks me about the MTBF rating of one of our devices. The abbreviation MTBF stands for

Mean-Time-Between-Failure

and indicates the reliability of the specified equipment. It is the typical time between failures for a specified device design — that is, the typical amount of time (in hours) any of a specified set of devices will function before failing.

However, different companies define failure in different ways, depending on the nature of the equipment and its function within a system. Also, test parameters and batch size are not standardized. Essentially, higher MTBF ratings for finished goods are obtained by building equipment with components that have high individual MTBF values — that is, better quality components.

MTBF grew out of the US military's attempts to formalize reliability assessment in the 1950s and 1960s which resulted in the publication of MIL-HDBK-217. Various flaws with this document led to a number of revisions and eventually, "... the U.S. Army has discovered that the problems with the traditional reliability prediction techniques are enormous and have canceled the use of MIL-HDBK-217 in Army specifications ..." Source: Equipment Reliability Institute's "ERI News", August, 2001 — vol. 4.

Despite criticisms of MTBF (especially within MIL-HDBK-217), it remains the dominant reliability assessment tool in the commercial electronics industry. The "Telcordia SR-332" handbook is used by many non-military electronic manufacturers for generating MTBF values. It evolved as follows: In the early 1980s Bellcore (Bell Communications Research) spun off from AT&T Bell Labs. Starting in 1985, Bellcore used MIL-HDBK-217, then improved and adapted it for highly-integrated commercial electronic products. In 1997 Bellcore was sold and its name was later changed to Telcordia Technologies.

At Contemporary Controls, equipment reliability is specified by MTBF values produced through the use of the Telcordia standard: Method I — Case I — Quality Level I.

Although the derivation of an MTBF value can be mathematically quite involved, the process can be generally stated as:

(Total Operating Time) / (Sample Size)

Suppose, as a very simple example, we test five electronic components until each one fails with the following results:

After totaling the above hour counts (3000), we would divide by the sample size (5) to get an MTBF for the component:

MTBF = 3000/5 = 600 hours

The above MTBF example means that we would expect the theoretically typical component to fail after 600 hours of operation. Stated differently, if we assume that all five components were typical, we would expect all of them to fail at 600 hours, with an average failure rate of one every 120 hours (600/5). Note that every component greatly outlived the 120-hour statistical failure mark for an individual. The 1-failure-per-120-hours is merely a statistical artifact that only achieves significance once the group size becomes much larger than in this example.

Actual MTBF values are much, much higher than the preceding example. Indeed, some exceed 1,000,000 hours! Industrial Ethernet switches usually have MTBF ratings of about 500,000 hours. That is, of all such units tested, the typical one would fail at 500,000 hours — also, all of them would fail at 500,000 hours, if the entire group is composed of typical devices. Of course, no one really tests devices for such a long time — 500,000 hours is about 57 years! Actual MTBF ratings are either: projections based on a record of actual product failures, or predictions made by aggregating known MTBF values from component or sub-assembly suppliers.

Some people like to look at the MTBF like this: If a group of 1000 Industrial Ethernet switches has an MTBF rating of 500,000 hours, we could expect all 1000 units to fail within some 57 years. But if all 1000 were placed in service over the same time period with an evenly-spread failure rate, we could statistically expect one to fail about every 21 days, based on the following calculations:

MTBF / population size = mean unit time to failure

(500,000 hours) / (1000 switches) = 500 hours mean lifetime per unit

(500 hours) / (24 hours in day) = 20.83 days

However, the foregoing result is very misleading. Firstly, assuming a symmetrically balanced failure record, the odds are 999 to 1 that a particular switch will fail after 21 days! Also, various factors (some unknown) skew the typical failure model toward the MTBF value. That is, in reality the 1000 switches tend to fail (or wear out) at roughly the same time (near the MTBF value). But the averaging process yields a statistical result that predicts one failure every 21 days — even though the true lifetime of the vast majority of switches is much nearer the MTBF.

From this you can see that an MTBF rating is of no value when applied to an individual item (nobody replaces a device every 21 days). Instead, the MTBF is a figure-of-merit that predicts the reliability of an entire group of products. What we should care about is: The greater the MTBF of the group, the more reliable a typical individual product within the group!

To Shield Or Not To Shield?

Occasionally I am asked, "Which cabling is better: shielded twisted-pair (STP) or unshielded twisted-pair (UTP)?" Both cable types have good inherent noise rejection because of the twisted conductors which act as a balanced transmission line.

STP works like UTP, but you seldom see it in normal networking (where better noise resistance is typically unneeded), because it is less flexible and more expensive. The issue has often fueled a debate between European advocates of STP and American supporters of UTP, but advocates of STP often tout its superiority without examining both sides of the issue.

STP shielding should be grounded at just one end to avoid ground loops (in case the ground potentials at each end of a link differ). These ground potentials are often unequal. In the rare case of an extremely noisy environment, grounding both ends of a link may be required for optimal results.

Since STP encases the signal wires within a conductive shield, you might expect all outside interference to be automatically blocked; but this is untrue. Like an antenna, the shield converts received noise into current which begets equal and opposite current in the signal pairs. If these two signal currents are symmetrical, they cancel each other and no noise is passed to connected equipment. But if the symmetry fails, the current in the cable becomes a source of system noise.

A prime consideration of STP is the need for close attention to proper grounding. STP only mitigates noise as long as the entire link is properly shielded and grounded. However, even a properly grounded system will eventually suffer grounding inconsistencies as equipment is added or is serviced or simply ages. And when the inevitable grounding inconsistency occurs, troubleshooting it can be difficult.

STP should work with equipment specified for UTP, but if you fail to ground the shield properly, the more expensive STP will not work any better and could easily perform worse since a poorly grounded shield will degrade the system by acting as a antenna for noise. STP will work with Contemporary Controls products.

ODVA 2006 in Arizona

The ODVA CIP Networks Conference and 11th Annual Meeting was held at the beautiful Pointe Hilton Tapatio Cliffs Resort (see photo) in Phoenix on February 22, 2006. Two Contemporary Controls attendees were R & D Manager Bennet Levine (right) and Software Engineer Harpartap Parmar (left). A good time was had by all.

Among the several presentations were ones on Real-Time Ethernet and on Designing Ethernet for Harsh Environments. Both of these papers have important insights into resolving tough issues.

Is Your CAT5 Cable Properly Paired?

I have received many calls about problems that were due to one of the most common issues facing network technicians: improperly installing twisted-pair cable. The problem of improper pairing presents mystifying symptoms that arise from crosstalk. The problem can be difficult to detect and cause corrupted or even total failure of data transfer. Perhaps the most common symptom is the failure of two ports to properly link with the Auto-MDIX feature.

Today, most twisted-pair LAN wiring is Category 5 (CAT5) unshielded twisted pair (UTP) cable. CAT5 UTP has 4 pairs, making a total of 8 wires. Each wire will have a single strand (solid) for running through walls and ceilings or multiple strands (flex) for patch and drop cables.

Pairs are color coded to make it easier to identify the same wire at each end of the cable. But more importantly, each pair uses the same color so pairs can be more readily identified from end to end.

Each of the 4 pairs has a wire of a solid color and its mate has the same color applied as a stripe over white insulation. EIA/TIA Standard 568B calls for the colors to be blue, orange, green and brown.

The most commonly used connectors with CAT5 UTP are RJ-45. The "RJ" stands for Registered Jack and "45" specifies the pin-numbering scheme.

Signals for 10BASE-T are carried on just two pairs: orange on pins 1/2 and green on pins 3/6. The other pairs are tied to pins 4/5 and 7/8.

The chart below illustrates the "right" and "wrong" way to wire an RJ-45 connector. The column entitled "Wrong Pins" depicts the common-sense pattern where each wire is placed adjacent to its pair partner. The "Right Pins" column shows the proper (but counter-intuitive) arrangement with the green pair split between pins 3 & 6. That is, you would expect the green pair to occupy pins 3 and 4 — not 3 and 6!