Michael Horowitz

When the video cable doesn't fit

January 03, 2010 6:45 PM EST

Once upon a time (OK, it was last week) while connecting a new monitor to a computer, I learned something the hard way about video cables and connectors. You can learn the easy way.

The computer, a Dell tower running Windows XP, was connected to the old monitor using an analog SVGA  (a.k.a VGA or D-sub) connection.  Video in the computer was provided by a PCIe based ATI Radeon X600 video card.

The monitor was not connected directly to the video card. Instead, it was connected to an adapter which, in turn, was directly connected to the external video port of the ATI card.  

THE PROBLEM

The new flat panel monitor had both DVI and analog connectors and cables. Since the monitor and the computer both offered a digital connection, upgrading from analog to digital seemed like a no-brainer. Yet, as I write this, the new monitor is connected to the same analog SVGA jack the old monitor used.

Why? The video cable didn't fit.

I asked a group of techies for help and got two guesses.

One theory was a physical problem. Perhaps when the male-ended video adapter cable was removed, a pin was left over, stuck in the female video card connector. Or, perhaps pins were bent on the DVI cable.

Good guess, but that wasn't the problem.

Another excellent guess was a mis-match between DVI connectors. As the DVI entry at Wikipedia shows, there are multiple types of DVI connectors.

The DVI standard supports both analog and digital, and comes in many flavors: DVI-D (digital only),  DVI-A (analog only), DVI-I (both) and M1-DA, which adds USB to the mix. Some of these flavors are then further sub-divided into single and dual-link. And, that's just normal DVI, ignoring both Mini-DVI and Micro-DVI.

Two helpful articles on this are All About DVI and DVI Connector Types.

According to the documentation, the new monitor came with a "DVI" cable. Thanks for nothing, Samsung.

As far as I can tell, monitors typically have DVI-D connectors and computers typically have DVI-I connectors.

Despite the many varieties of DVI connectors, this was not the problem.

A CLUE  

It turns out I was ignoring a big clue, because I didn't know it was a clue: the video adapter was two-headed.

That is, the adapter was shaped like a Y. One end connected to the computer and the other end had two SVGA connectors.

Hats off to anyone who gets the significance of the clue.


THE ANSWER

The reason the DVI cable included with the new monitor didn't fit, is that the connector on the back of the computer was not DVI at all. Neither was it VGA or S-Video. It was DMS-59.

The two headed cable is a clue, because a DMS-59 connector is designed to support two monitors. Thus, most DMS-59-to-something adapter cables are Y shaped.

Like DVI, DMS-59 supports both analog and digital signals. Some DMS-59 adapters have two analog SVGA connectors, others have two DVI-I connectors (the type that does both digital and analog). I even ran across a DMS-59 adapter with one of each.

Wikipedia warns that "Some confusion has been caused by the fact that vendors label cards with DMS-59 as "supports DVI", but the cards do not have DVI connectors built-in."

Good to know when buying a video card.

The next sentence hit the nail right on the head:  "Such cards, when equipped with only the VGA connector adapter cable, cannot be connected to a monitor with only a DVI-D input."

Bingo. A new DMS-59 to DVI-I adapter cable is in the mail.

How widely used is DMS-59?

None of the handful of techies I ran this problem by had ever run across it. My 17th Edition of Upgrading and Repair PCs by Scott Mueller (circa 2006) has nothing on DMS-59. This is a 1,400 page book devoted to PC hardware that covers DMA, DMI, DMM and DMT - but not DMS .

Yet, the day after this experience, I was in someone's office staring at the back of another tower computer. The flat panel monitor on the desk was connected to an SVGA port on a two-headed adapter cable. No doubt, another DMS-59 sighting.