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ARCHIVE >  August 2013 Issue >  Electronic Mfg. Services > 

Sorting Through Three Grades of D-Sub Cable Assemblies
Connectors used in "premium" grade D-Sub cable assemblies have knurled thumbscrews for ease of plugging and unplugging, conductors that are never thinner than 26 AWG, and a double shield.

D-Subminiature cable assemblies have given way to lighter, faster, and less expensive cables in many standard applications, but D-Sub cables, as they are known, are still widely used in a variety of systems. They are generally more durable than smaller cables, using large, rugged D-Sub connectors that stand up well to abuse. Available with connector pin configurations ranging from 9 through 78 pins, D-Sub cable assemblies are easier to apply to custom and home-grown applications (compared to standardized protocols) and are very easy to rewire by simply cutting off one connector and terminating a new one. With their locking screw hardware, they remain securely mated even when exposed to shock, vibration, and sudden pulls on the cable.

Those accustomed to collecting quotes for other commercial-off-the-shelf (COTS) cable assemblies, such as Universal Serial Bus (USB), Ethernet, or coaxial cable assemblies, may be surprised at the wide range of prices when comparing quotes for D-Sub cable assemblies. Are these differences because of variations in company pricing strategies, or are other factors involved? As it turns out, a number of different factors contribute to the wide range of prices for D-Sub cable assemblies. These cable assemblies can be broken down into three different "grades," ranging from the most basic and least expensive to the highest quality and most expensive. Understanding the needs of an application can help match the right grade of D-Sub cable assembly to that application every time.

Premium Cables
What differentiates these different grades of D-Sub cable assemblies? The highest cost of "premium" D-Sub cables provides numerous enhancements to the overall cable construction. They feature easy-to-grip knurled thumbscrews to simplify plugging and unplugging, and they are easily replaceable for special mating circumstances. The backshell is either cast alloy and assembled by hand, or encased in an internal steel box to protect the pin terminations, then overmolded. Bulk "premium" grade D-Sub cables have a double shield, usually 100 percent foil wrap plus a tinned copper braid, typically soldered a full 360° to the metal in the backshell to ensure drainage. For this grade of D-Sub cables, the wire gauge of the conductors is never thinner than 26 AWG, and can be as thick as 20 AWG to carry higher electrical current.

In terms of performance, why use a "premium" grade of D-Sub cable assembly? Demanding requirements, such as in environments with exposure to EMI and RFI, especially on longer cables, could necessitate the double shield and 360° shield drain solder used in these "premium" grade D-Sub cable assemblies.

Applications which require frequent plugging and unplugging, vibration, constant movement or occasional rough pulls or impacts can rapidly degrade the condition of a cable assembly, and the thicker conductors and high-grade thumbscrews on "premium" D-Sub cables can endure such conditions better and longer than the materials and configurations of the other D-Sub cable assembly grades. Also, if an application requires extremely high uptime (95 percent or higher), as in process automation or safety control applications, such performance is more typically achieved with "premium" grade D-Sub cable assemblies. Such cable longevity translates into less of a need for in-house replacement cables (and their associated costs), although the price for the cables in use may be somewhat higher than the other grades of available D-Sub cable assemblies.

Deluxe Grade Cables
The "deluxe" grade of D-Sub cable assemblies may lack the full ruggedness and performance of the "premium" grade cables, but meets the needs of many applications. Normally, this grade of D-Sub cable assembly has only a foil shield, with conductors ranging from 24 to 28 AWG. The terminations in the backshell typically use 360° copper foil shielding for additional protection, with the shield drain wire soldered directly to the connector shell. The mating hardware is normally commercial-grade thumbscrews that may be easily removed and replaced to suit different applications.

Such adaptations are actually quite modest compared to the "premium" grade cables, and only slightly reduce cable performance, but they can have a dramatic effect on price. "Deluxe" D-Sub cables sometimes cost one-half as much as their "premium" D-Sub cable counterparts. The "deluxe grade of D-Sub cable assemblies will not meet the requirements of the most demanding applications. But for almost any other application, this grade of D-Sub cable assembly can provide excellent and reliable performance. It may even make sense to keep excess stock in-house because they can be used in so many different applications.

Economy Grade Cables
For the least demanding applications, and for cases where the cable will be plugged in and left for a long time, perhaps until failure when it will need to be replaced, the least-expensive or "economy" grade of D-Sub cable assembly may be the grade that makes the most sense for this type of application. These cables would have a 100 percent foil shield, and the conductors are no thicker than 26 AWG. The backshell is typically molded over a 360° foil shield, with slotted machine screws that require a screwdriver for attaching and unplugging. In terms of cost, however, the cost of an "economy" grade D-Sub cable assembly can be 60 percent less than its "premium" grade counterpart.

These "economy" grade D-Sub cable assemblies should only be used in situations where the environment is stable, meaning the cable is rarely moved or unplugged, and the EMI/RFI is reasonably low. Often, cables in these applications are shorter, minimizing the exposure to EMI/RFI. Such applications are generally in clean places, like homes, offices, or clean point-of-sale (POS) counters, and be plugged in behind a personal computer (PC) or serial device that rarely moves. Because of the low cost of this grade of D-Sub cable assembly, it makes sense to store a few extra cable assemblies in-house as replacements in case of failure. For any of the three grades of D-Sub cable assemblies, the needs of the application will generally provide any guidelines required for choosing which cable grade is the best fit for that application, and whether or not backup extras should be kept on hand.


Contact: L-Com, Inc., 45 Beechwood Drive, North Andover, MA 01845 800-341-5266 E-mail: sales@L-com.com Web:
http://www.L-com.com

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