Making the Right Connection
How to choose the best interface for your purposes
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Helping to drive interoperability between computers over networks, industry standards have facilitated the growth of local area networks (LAN), storage area networks (SAN) and the Internet. The virtue of standardization is that much of the time-consuming engineering is eliminated and, along with it, delays and headaches, since cables, connectors, pinouts and electrical issues are predetermined. Unfortunately, there are now so many standards, that designers face major challenges in picking which interface to build into new devices.
Standards for physical layer interfaces — which define electrical, mechanical and signaling transmission characteristics — have been evolving since the 1970s.
• Work on Ethernet started in 1973 and has been in use since 1980.
• Small computer system interface (SCSI) first appeared in 1981 and is still in widespread use in the enterprise storage space with its high-density, 68-position tab and receptacle connector.
• Universal serial bus (USB) took its first steps in 1996 and, now in version 2.0, is still the de facto interface in personal computers and many peripherals with its four-position connector.
• Infiniband, a merging of two competing standards, saw adoption among the industry’s leading companies making high-performance information communications technology (ICT) beginning in 2000. Infiniband created yet another type of connector, the multi-lane serial attached SCSI (SAS) or 4X (SFF8470).
• SAS emerged in the early 2000s and, in its latest iteration, miniature multi-lane SAS, or miniSAS, is delivering almost double the data rate of its predecessor in an even smaller but, again, different connector — one for internal connections using 36 pins and another for the external input/output (I/O), which uses 26 pins.
With so many choices, how does an engineer pick the right connector for his or her purposes? In many companies, that decision may be driven by the marketing department which, one hopes, has researched their customers’ needs or made a risk assessment about emerging or immature technologies. Yet, even industry standards do not stand still; they continually reiterate and re-emerge as new and improved revisions.
Continuous improvement means updates, upgrades and the problem of connecting legacy equipment. The result is myriad types of adapters, cascade cables, extension cables, jumpers and leads, which merely pass the headache down the line to the technicians in the data center who will have to install, maintain and repair equipment over the course of its life.
Returning to basics
While connectors and cables are vitally important components, they often are the last thing to be considered in new equipment designs. Yet, they are the one group of components customers regularly handle during the lifetime of using a product. With this in mind, there are several important attributes to consider in evaluating these components:
— Suitability: Obviously, the needs of a user of a handheld computer differ from those of an IT manager running a supercomputer. As such, the design of connectors and cables should reflect the differences in intended applications, including the following:
— size
— weight
— ease of connect and disconnect
— ruggedness
— ease of replacement or repair
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Fortunately, most standards bodies, such as the Institute of Electrical and Electronics Engineers (IEEE), the American National Standards Institute (ANSI), and the International Electrotechnical Commission (IEC), have given consideration to these factors. Also, where an industry standard does not exist, there may be a de facto standard that has been established by a leading manufacturer or consortium. Some organizations host “plug fests” to encourage interoperability, where participating manufacturers attach their devices to other manufacturers’ and look for bugs.
— Industry standards: As we have noted, there are many standards. Picking the right one for your design calls for both technical and business judgment. It is important to note that not all industry standards explicitly define all aspects required of connectors and cables. For example, a standard may set out dimensions and pin assignments but omit the precise details of contact design or latching. In this situation, it is wise to consider products sold by leading manufacturers and to review their technical literature for similarities and differences, as well as to consult with customers about use expectations.
Standards committees are continuously reviewing new technologies and looking at ways to extend the reach of their specifications or their suitability for new applications. For this reason, it is important to be are aware of the current status — details can and do change.
If you intend to tweak a standard — as changing the wiring configuration — to suit your platform, there may be consequences for interoperability with devices that fully comply with the extant standard. You also may be limited in compliance claims you can make for your platform.
— Cable considerations: Available in a bewildering selection of styles and constructions, not to mention gauges and colors, differnet types of cable offer different performance characteristics. However, cable manufacturer’s technical specs can help you to find the best combination for your equipment. Be sure to compare the following:
— shielding effectiveness
— skew
— impedance
— attenuation and cross talk
Flat cable is attractive because it is cost-effective and easy-to-use. Unfortunately, common unshielded flat ribbon cable, such as ribbon or multi-conductor cable, is not well-suited for high-speed differential signaling (except for very short runs). For low-voltage differential signaling (LVDS) applications, balanced cables, such as twisted pair, are usually better for noise reduction and signal quality than unbalanced cables, since balanced cables tend to generate less electromagnetic interference (EMI) than unbalanced. They also tend to pick up electromagnetic radiation as common-mode (not differential-mode) noise, which is rejected by the receiver. Shielded ribbon cable is particularly well-suited for high-speed differential signaling, such as data transfer between servers and storage devices.
— Shielding requirements: Different applications have different shielding requirements that are determined by the end use for the device, as well as by the final country where it will be purchased. Bear in mind that compliance to a country or region’s regulations and directives is mission critical, and properly designed shielded I/O connectors and cables should play an important role in strategies and programs for electromagnetic compatibility (EMC).
— Download technical specifications from component makers’ Web sites (or call and request to see hardcopies). Include the specs in your technical construction file.
— To ensure the wire bundle is adequately covered, pay attention to the construction and quality of cable braid and foil shields. (Ideally, it should be 360-degree coverage).
— Examine junction shells for potential hotspots. Metal shells should completely cover the cable/connector interface, while plastic or molded shells should contain an inner metal or foil wrap.
— Well-designed components will drain away electrostatic discharge (ESD) before the connector contacts wipe, protecting the sensitive circuits of the equipment. As a result, it is important to consider the design of the protective metal face or shell on the mating side.
— Wiping contacts: Connectors broadly divide into two types based on the design of the wiping contacts: — Pin/socket — The best-known pin/socket connector is the D subminiature, or D Sub, invented by ITT Cannon in 1952. It is still used for RS-232 serial communications. In its half-pitch iteration, it was adopted as the SCSI-2 and SCSI-3 connector. However, in .050-inch pitch and smaller, the pins become fragile and more prone to breakage.
— Ribbon-style — Most high-density I/O connectors are of the ribbon-style design. The precise geometries of the pre-loaded ribbon contacts ensure reliable connection, even in miniature sub-1mm versions, while the supporting insulator reduces the risk of damage during insertion and withdrawl.
Consideration also should be given to contact plating. In order to protect the copper alloy contact from wear or damage due to oxidation, gold is widely used as a plating material. Gold also reduces the insertion/withdrawal forces.
— Higher gold plating levels — Typically 30µ or higher, this type of plating is preferred for high-value electronic ICT equipment with long service life.
— Selective plating — Typically 10µ or lower) this plating is better suited to short-life consumer electronic devices.
— Mechanical fixtures: Selection of mechanical fixtures is driven by the application. In central office or data center environments, latches may suffice, such as the lever found on RJ-45 connectors or the squeeze latch found on many ribbon-style I/O connectors. However, in high-vibration environments, such as factories, thumbscrews are recommended, and the industry standard may specify mechanical hardware down to thread size. Bear in mind that different regions of the world have preferences for one size over another.
— Ease of repair: Consider aftermarket programs to ensure that the right designs and qualities of cable assemblies are available to system integrators and equipment installers from their distribution partner of choice, be it a retailer, a cataloguer or direct from the company.
Choose wisely
If you fail to consider connector design, your technical help line could be inundated by irate technicians. How one device or sub-assembly is connected to another is critical to ICT equipment design. Select the best cable construction for your equipment, and the system will live up to its literature claims and allow customers to enjoy years of service from their investment.
One final point — resist the temptation to buy the cheapest product. There’s usually a design or material savings that makes it the cheapest. Shaving a few cents from the cost of a plug connector cable might be tempting. However, if a breakage leads to a service call request, the true applied cost will dwarf any savings. As in all things, you get what you pay for.
P. Lindsay Powell is business development manager in 3M’s Electronic Solutions Division. He may be contacted at editor@ScientificComputing.com.
Acronyms
ANSI American National Standards Institute | EMC Electromagnetic Compatibility | EMI Electromagnetic Interference | ESD Electrostatic Discharge | I/O Input/output | ICT Information Communications Technology | IEC International Electrotechnical Commission | IEEE Institute of Electrical and Electronics Engineers | LAN Local Area Network | LVDS Low-voltage Differential Signaling | SAN Storage Area Network | SAS Serial Attached SCSI | SCSI Small Computer System Interface | USB Universal Serial Bus
