Looking to the Past for Today's High-Reliability, Low-Cost Connections

The perfect connector for your military, medical, instrumentation, and industrial application may not always be the connector based on the latest technology. Telecordia-qualified connectors developed for the telecommunications industry are widely available from many manufacturers and can fit snugly into all of those high-reliability applications. To understand why and how, let’s take a look at the tried-and-true telecom connectors.

Connections with the Past

Telecommunication is different from markets like computing and storage in many ways. Up until 1982, the telecom networks around the world were generally government-controlled monopolies. In the U.S., AT&T had a complete monopoly, controlling every aspect of the network, all the way down to your hard-wired telephone. AT&T even owned the phone and leased it to the subscriber for a monthly fee.

Leasing to a very large base provided a very strong motivation to have extremely high reliability levels, minimizing service and maintenance costs. If you pick up one of these pre-1980s phones, you immediately note the considerable heft. There was also a strong premium on long life. Every network element, even the phones, was designed to last for 40 years in trouble-free service. Phones were hard wired with screw-down lugs or solder connections and the connections in the network were slathered with 50 micro-inch gold plating -- a level a jeweler would love.

AT&T had a laboratory arm called Bellcore that wrote specifications for central office architecture that standardized all the racks so that new equipment could easily fit into the existing racks. These racks were passively cooled without using fans (points of failure). In those days, individual wires from your phone were aggregated into larger cables inside those green pillars you still see in your back yard. Those cables were routed to a neighborhood patch panel, with larger copper cables to the central office. Arrays of 50-pin, telco connectors covered the rear of the telco racks. Central offices were convectively cooled without fans, limiting electronics density, but still quite sufficient for voice lines, highly reliability, and cost effectiveness. (And, by the way, quiet.)

Bellcore and Bell Labs were responsible for several new technology developments that have enabled the modern network to evolve:

• Compliant pin press-fit for backplane pins: They used square pins that were forced into a 0.040” round hole. These pins were plated 100% with 50 micro-inch gold plating.
• Insulation displacement technology for solid conductor twisted-pair cables enabled efficient mass termination of the 25 twisted-pair cables that were ubiquitous in the central offices.
• It was unacceptable to turn off a large number of customers to fix a problem with one customer. This lead to hot-pluggable cards and redundancy schemes that kept customers online.
• To compensate for potential power failure, central offices still have large battery plants (the lead acid kind.) The racks are fed with 48-volt power and down-converted on the cards with power conversion chips like those from RECOM and Lambda Electronics. The relatively high-voltage, low-current approach enables most cards to be served with simple 3-5A per circuit power connectors with multiple mating levels to facilitate hot swapping. Central offices now use these power blocks, developed for telcos, across almost all backplane connector applications.
• By 1990, the operating companies realized that they needed more density and lower cost than could be supported by 0.100" square-pin technology so they developed a breakthrough specification, Bellcore TR-NWT-001217. For the first time, a specification existed that provided test procedures enabling qualification of smaller pitch connectors for use in these 40-year life networks. A frenzy of development followed that has resulted in the variety of high performance, high-density backplane connectors we enjoy today.

Access to the Network Then and Now

The access network of the past (and now) is similar to a snow flake with large fiber pipes between the central office and patch panel nodes found in neighborhood boxes or commercial buildings. Lower-speed copper connects the nodes to subscribers.

In the past, separate companies delivered voice and video. Voice service came from the phone company over a pair of 22 AWG, differential pair conductors comprised of four wires with no ground shielding. These cables deliver voice plus 48 V power to keep your phone working even when the power from the electric power utility has failed.

Cable companies worry about attenuation over a broad range of frequencies and use the rather bulky RG-6, 75-ohm, conductors with 18 AWG wire and F connectors. The more subscribers that share one cable, the lower the bandwidth available for each consumer.

Today's networks are quite different. Telcos and cable companies both deliver a broad range of services including VoIP, data, and video. The network is fiber up to the last mile where it is converted to the form that reaches your home in the following ways:

• Twisted pairs from telcos
• RF coax from the cable company carries multiple frequencies simultaneously on one wire
• Fibers from advanced networks, generally in dense urban environments
• WiMAX allows wireless connections to the home

The common denominator is packetized data that is multiplexed at the node, and split out at the customer’s residence to serve all of the communication needs in the home.

Intense competition has enabled us to now enjoy voice, video, and internet over the same pipe – leaving us with energy to spend sorting out the idiosyncrasies of the home network and devices within the home. Further, the traditional and cable providers now find themselves competing more and more with Apple, Google, Microsoft, and many more. i

This level of service has changed the central office dramatically.

• Fiber has replaced individual twisted pairs
• I/O has moved to the front panel
• Larger, deeper chassis with cooling fans enable much higher electronics density making telco central offices to look very similar to computer data centers
• Routers have replaced switches, allowing Cisco and others to gain share from traditional Telco equipment suppliers. And routers are smart. Voice, data, and video packets are consolidated in one stream from the central office. The router looks at the address on each group of packets and sends the info to the appropriate end device. Central office routers multiplex massive data streams on to long-distance fibers with laser transmission

One common denominator between the old network and the modern one is the use of backplane architecture, with bookshelves of IO cards that plug into a passive multilayer backplane using high pin count connectors. Prior to the mid 80s, telco backplanes were dominated by 0.025" square-pin technology. European telco suppliers used three-row, 96-pin DIN connectors and AT&T used proprietary connectors. The DIN connectors are still used very broadly across many industries. These are open-pin field connectors. Individual pins can be designated for signal, power, or ground as needed for the application. As signal speed on an individual pair increases, additional grounds are added, providing a level of isolation and ground reference.

The DIN 41612 specification standardized pin and housing dimensions and configurations, enabling many suppliers to provide parts that inter-mate and work well together. This open competition for standard connectors has made these DIN 41612 backplane connectors the solution for high reliability and low cost applications. Many different variations are available off-the-shelf to fit a wide range of telecommunications, medical, instrumentation, and industrial applications. These are available from manufactures including AVX, FCI, Harting, Hirose, Molex, TE Connectivity, and others. The 2A current rating for 0.025" pins means that separate power connectors are not needed for the majority of applications. Many of the DIN standard blocks include provisions for screw-machine RF and high-power connections like those from FCI, making them well suited for heavy-duty industrial and military applications.

Beyond DIN

Industry standards like Multibus and VMEbus are built around standardized card cages and protocols enabling a systems engineer to mix cards from multiple vendors, creating a system ideally configured for the application. Both of these protocols use a backplane bus architecture where each card sees each signal on the backplane. The low-cost, compact form factor and simplicity has made this architecture quite popular for a long time.

New, finer pitch connectors were developed in the early 1990s as the telcos drove to higher density connectors. The Futurebus 2mm pitch connectors such as those from FCI are modularized in n x 12mm blocks, enabling the system designer to configure the backplane connection with as many or as few modules needed for a specific card. The Futurebus connector family includes discrete guidance and power blocks that make it quite easy for a design engineer to configure a system using standard off-the-shelf building blocks.

The 2mm Hard Metric (HM) connectors with n x 25mm blocks were also standardized and adopted broadly across markets and applications, supplanting DIN in many applications. These building block connectors are highly reliable, broadly available from many suppliers, including ERNI Electronics and JAE, Electronics, and Harting and are less expensive than the 0.100" pitch connectors.

The PICMIG CompactPCI® architecture uses card cages similar to VMEbus, but with 2mm HM connectors, making higher capacity systems in the same form factor. Engineers designing systems in this format can buy a plethora of boards from a variety of suppliers and can upgrade with new boards when cards with better chips come on the market. This allows systems to have I/O and processor cards, storage, and DSP capabilities best suited for the application. These standards are broadly used in medical, military, and industrial as well as telecommunications applications.

As you can see, the combination of legacy backplane connectors with industry standard architectures provides a broad base of solutions for the system designer. For maximum ruggedness, reliability, and current carrying, consider the DIN 41612 connector family. This is most ideal for military and heavier industrial applications. For good reliability combined with higher density of interconnect, the 2mm Futurebus and 2mm HM families are excellent solutions. The building block nature of these families is well suited for a wide variety of telecommunications medical, instrumentation, and industrial applications. Mouser Electronics has thousands of different connector part numbers on their shelves, with new connectors arriving all the time. And if one of the over half a million connectors that Mouser offers is not in stock, you can order it with a known lead time, or view delivery dates from on the site. You will find the optimum set to meet your needs and have the connectors quickly available for prototype builds as well as production.

The old model is that the telco or cable company owned the pipe that brings content to the home. Content providers, movie and TV show producers, etc. package content through “channels” and the utility sold packages to the consumer, with the price based upon the value the consumer was willing to pay. The new model is more complicated. The internet is becoming the pipe. The utility still gets paid to deliver the internet, but not much. Companies like Netflix, Hulu, Apple, Google and Microsoft now sell subscription services that include content, effectively cutting the utility out of the picture. Add the TV everywhere model over WiFi, and the utility no longer delivers content to the consumer. This is an interesting competitive and regulatory issue, but beyond the scope of this article.