Dell has a great computer series with its Optiplex line of business computers. Dell offers several form factors for each model and all models share -- for the most part -- drivers and capabilities. While the larger Opties allow expansion, the smaller form factors save space. The form factor oddball in the Optiplex 745, Optiplex 755, and Optiplex 760 line-up is the Ultra-Small Form Factor or USFF machine. These petite boxes lack some features that their other form factor cousins have, but Dell takes great strides to provide nearly the same capabilities for this pint-sized computer.
Dual Monitors with USFF Optiplex Computers
The focus of this article will be video, and specifically setting up dual monitors (single desktop) on Ultra-Small Form Factor Optiplex computers. The other form factors support video cards, but USFF Opties do not - and they are given only Intel integrated graphics and a single DVI video port connector. However, Dell engineers have gone to considerable lengths to provide the ability to provide dual monitor capabilities from that single DVI port.
DVI Supports Analog and Digital Signals Over One Wire
First a little background we need to understand that DVI connector. If we take a look at Wikipedia, we learn that the Digital Visual Interface pin-out contains pins for digital connections (pins numbered 1 through 24) and separate pins (numbered c1 through c5) for analog signals.
Normally, if a video card pushes computer video through a DVI connector, it pushes the same screen to both the digital and analog portions of that DVI connector. That way, an owner can connect to a DVI-capable monitor directly, or add a simple DVI-to-VGA adapter and connect to a VGA monitor. Note that in both examples, one set of pins is ignored by the monitor and inaccessible to other monitors.
Dell Magic: Two Signals from a Single DVI Port
But Dell engineers did something ingenious to allow dual monitors from a single DVI port. They created a Y-splitter cable that attaches to the DVI port on the computer, and routes the analog pins to one VGA connector and the digital pins to a separate DVI-D (more on that later) connector. When two monitors are connected, the BIOS sends different monitor signals to the analog and digital pins of the single DVI port on the back of the USFF Optiplex machine! With the Y-Splitter-Cable, the owner can connect one VGA and one DVI monitor and reap the benefits of a dual-monitor-single-desktop setup. In this respect the USFF Optiplex has dual-monitor capabilities similar to the other form factors (that rely on video cards to accomplish this) in its family.
Why You Can't Configure Dual VGA Monitors
Now comes the fun part and the reason why I am writing this article. If you have two VGA monitors -- you can't obtain the dual-monitor-single-desktop setup. At first glance it seems completely possible. All I would need to do is get an inexpensive DVI-to-VGA adapter, attach it to the DVI portion of the Y-Splitter-Cable, and we're done. But there's a problem.
Let's look closely at that Y-Splitter-Cable connector. At one end we have a male DVI-I connector to attach to the DVI-I port on the back of the USFF Optiplex box. DVI-I connectors are "all the pins" meaning both analog and digital. At the other ends of the cable are two connectors, a female VGA, and a female DVI-D. The female DVI-D connector lacks analog connections. And rightly so, because as we see above, the C1-C5 analog pins at the computer have been routed over to the VGA side of the Y-Splitter-Cable; there's no analog signal left to send to this side, so only the digital pins are available for connection.
All I Need is An Adapter...Right?
I had naively assumed that I could attach that inexpensive DVI-to-VGA adapter (that I mentioned above) to the DVI side of the Y-Splitter-Cable and attach my second VGA monitor to the adapter. The problem is both physcially and technologically impractical.
The inexpensive DVI-to-VGA adapters are all DVI-I (both analog and digital) males to VGA (analog only) females. This makes perfect sense. Those adapters route the analog C1-C5 pins to the analog RGB portions of a VGA female adaptor. But the Y-Splitter-Cable has already routed those analog computer video signals to the other half of the Y-Splitter-Cable. The DVI-D side of the Y-Splitter-Cable lacks the c1-c5 analog pin-ins. The inexpensive DVI-I male connector has c1-c5 pins and therefore cannot connect to the female DVI-D cable connector. This is the physical barrier. Please Note: There is no such thing as an inexpensive DVI-D-to-VGA adapter. You might see some advertised on eBay like I did and be tempted to buy one like I did. What I received was actually a DVI-I-to-VGA adapter despite claims and photos of the seller. But even if somehow there was such a connector, it would be trying to route non-existent analog signals to your VGA monitor.
Now for the technology challenge. At the DVI side of the Y-Splitter-Cable, we have only digital pins available for connection. In order to get a VGA signal out, we would need to convert the digital signal to an analog signal before getting it to a VGA monitor. Conversion from digital to analog is possible, and there are products that do this -- but they are about as expensive as a new monitor with a DVI connection and therefore impractical from a cost standpoint.
There you have it. From a single DVI-I port on the back of a USFF Optiplex compter, Dell has allowed dual monitor capabilities, but only when one is VGA and the other is DVI. Trying to attach two VGA monitors is not practical.