D-Subs Continue to Lock Down Dependability

Like the 40-ish professional athlete, D-Sub connector technology is no longer considered fast or sleek, but it is still going strong for one reason–it gets the job done.

BY STEVE SMITH, L-com

The D-Sub connector interface has survived being replaced by newer, faster, more compact interfaces. USB, among others, has replaced D-Sub serial and parallel ports on computers and printers. And DVI, HDMI, and the new DisplayPort interface have started to compete with SVGA (HD15 D-Sub) as the primary video interface on computer monitors as well. Nevertheless, D-Sub survival is due to several product features:

• Engineers prefer the lock-down mating hardware between screw and standoff to the non-secure retention of USB, Firewire, HDMI, etc. Simply put, D-Sub hardware can be permanently locked down and will not come loose due to vibration or jostling.
  
  
• Standard D-Sub connectors range from 9-position to 50-position, and high-density D-Sub connectors allow for up to 78 positions, giving designers flexibility depending on how many signals are required in their application.
  
  
• Costs for connectors and cable assemblies are reasonably low.
  
  
• Connectors and cable assemblies are available with many options. Printed circuit board (PCB) connectors are available as through-hole or surface-mount, can be stacked, and are also available filtered for high electromagnetic interference/radio frequency interference (EMI/RFI) environments. Cable connectors have multiple options for termination: solder, crimp/poke, insulation displacement, wire wrap and mass termination. Also available are combination connectors that reduce the number of signal contacts to accommodate coaxial, high current or high voltage contacts.
  

FIGURE 1. D-Sub connectors remain popular in part because they can be permanently locked down and will not come loose via vibration or jostling.

Although they are called D-Subminiature, by today’s standards, these connectors are anything but miniature. Most designers look to reduce the size of their products, so valuable real estate is taken up quickly with D-Sub connectors.

FIGURE 2. Contacts are arranged in parallel rows varying from 2 to 4 rows depending on connector size.

In addition, because D-Sub connectors are connected to an unlimited variety of cable, there is no defined speed that the user can rely on without calculating and/or testing individual scenarios. Also, with the exception of some custom cable assemblies using larger AWG conductors, D-Sub connectors are considered signal–not power–connectors. Therefore, they do not readily and economically combine transmission of power and signal as do USB and Firewire.

D-Subminiature connectors were developed in the 1950s, and are distinguishable by their “D” shape, which ensures correct orientation. When mated, the female connector fits snugly inside the male connector. Because D-Subs are metal, a continuous, 360° shield can be created throughout the entire cable assembly and connector system.
Contacts are arranged in parallel rows varying from 2 to 4 rows depending on connector size. There are five basic shell sizes–standard contact arrangements range from 9 to 50 positions, while high density versions range from 15 to 78 positions.

FIGURE 3. In one D-Sub application, L-com helped a company design daisy-chain assemblies containing several smaller cables, all wired into larger 37- or 62-pin D-Sub connectors.

Industry-standard commercial contacts are stamped metal while military-rated contacts are screw machined. Contacts are typically gold-plated–anywhere from 3 microinch gold flash to 30 microinch, depending on how many mating cycles they are expected to withstand.

Current Applications

Jim Smith, production engineer at HBM Somat (www.somat.com), a provider of rugged, portable data acquisition systems, states D-Sub connectivity is used throughout the company’s product lines because of its “reliability and convenience.”

Thirty-two signals are required to wire Somat’s thermocouple PCB to a cold junction compensation box. Somat designed in a standard, off-the-shelf 37-position D-Sub cable assembly to make the connections. For more complicated interconnet challenges, Somat designed a few daisy chain assemblies containing several smaller cables, all wired into larger DB37 and HD62 connectors (Figure 3). These cables make assembly easy, allow for multiple interfaces to be used, and take advantage of D-Sub’s high contact count and secure connection between cable assembly and PCB.

FIGURE 4. An L-com solution (L) for one company helped eliminate the problem of backshells that can’t provide adequate strain relief due to the non-uniform shape of the cable bundle, and individual cables occasionally pulling out of the backshell strain relief (R).

Another benefit of utilizing D-Sub connectors in these cables is the ability to mold the backshell, resulting in an exceptionally durable product. Initially, Somat had the cables manufactured with assembled/mechanical backshells. But because these assembled backshells can’t provide adequate strain relief due to the non-uniform shape of the cable bundle, individual cables occasionally pulled out of the backshell strain relief.
L-com proposed a molded backshell solution, which has eliminated this problem and provided a more professional appearance (Figure 4).

Tom Baker, electrical design engineer at Bally Technologies (www.ballytech.com), a manufacturer of casino games and slot machines, also uses D-Sub technology in a variety of applications. According to Baker, Bally uses well-shielded SVGA cable assemblies featuring HD15 connectors to “pass stringent EMI emissions requirements.” Bally also uses serial RS232 DB9 cables for their touchscreen interactive video games, some of which utilize bulkhead-mounted, dual-sided adapters to pass cables through enclosures.

FIGURE 5. To help a company meet stringent shielding requirements, L-com designed a cable assembly that provides enhanced shielding, with the shell connecting the braid to the D-Sub connector and ensuring 360° shielding throughout the assembly.

Matt Nitka, lead engineer for a medical product manufacturer, shares Bally’s use of 9-pin D-Sub serial RS232 cables for touchscreen monitor displays. Also similar to Bally but with serial cables, Nitka states they must pass rigorous UL emissions demands and require 360° shielding with negligible EMI leakage.

Industry-standard cables use a foil/mylar shield and carry this shield from connector to connector via an electrically connected drain wire. These cables may or may not shield the area under the backshell mold. L-com’s premium line of cables, however, adds a tinned copper braid shield over the foil/mylar shield and a steel internal shell under the overmold. As shown in Figure 5, the braid provides additional shielding and the shell connects the cable braid to the connector, ensuring 360° shielding throughout the assembly.

A Versatile Solution

With numerous applications taking advantage of the attractive features of this connector system, D-Sub connectors appear to be in no danger of extinction. They are simply too versatile, reliable, and offer many options for passing signals from point A to B.