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When I was tidying up my garage I found an old 386SX laptop with an 8" passive LCD screen. After thinking really really hard and failing to find a use for it, I decided that I'll take it apart and then hook up the passive LCD screen to my linux box and have it sit there scrolling web server logs. Would that not be impressive? I think so. A reasonably complex idea, especially keeping in mind that I would have to rewire the 15-pin VGA plug and hook that up to the LCD inputs.

Then I started to think further, and did some research on the topic. What I found out didn't exactly impress me.

Generally, it is considered impossible to connect an LCD screen to a VGA card's output. The way that LCD screens work is that they have graphics controller chips (a Cirrus Logic one in my case) that have two output ports:
  • One for external analog VGA output.
  • The other for digital control of the LCD.
Graphic LCDs are notoriously non-standard and, even to the extent that they are becoming more standard, were horribly non-standard during the time at which most 386 and 486 laptops were made. So the main problem here is that VGA cards have an analog output, and the LCD screens have a digital input.

With my no-brand LCD panel and a random LCD controller device that I wanted to talk to the VGA card, I was in for a few days worth of staring at a logic analyzer and hacking driver code. Problem made even more difficult by the fact that I don't even own a logic analyzer.

As I said above, it is generally considered an impossible task. With enough motivation and money everything can be accomplished, and this is no exception. To connect a digital LCD screen to an analog VGA interface you'll need a mixed-signal microcontroller communicating with the panel to translate the analog VGA inputs into the digital signals used in the LCD. There's some pretty complex circuitry involved in this. The commercially available 15-18" LCD panels with SVGA inputs do exactly that, but again, they are expensive and mostly feature proprietary designs. And don't forget that you will also need to write a driver for the newly created hardware.
So basically I gave up on that idea. If anyone knows a reasonably simple way or doing this conversion, please post it. Otherwise, "Top Ten Things To Do With An Old Laptop" would be a node that I will soon have to visit.

...not as impossible as you may think, my dear boy. It is possible in some cases. Though whether or not it's worth the considerable time it takes is up to you to decide.

Although Laptop LCDs use special ASICs to function, even these have to conform to some sort of industry standards. Surprisingly enough, more often than not many panels fron different manufaturers will conform to the same specification.

Then you need a way to get your PC at home to actually drive the LCD in question. And this is where you have to spend some money. Earth Computer Technologies (www.flat-panel.com) makes a nice line of LCD controller based Video Boards just for this purpose. They have many boards so that if you're willing to to pay the cash, you can drive the panel. While their cards are on the steep side, they are (at the moment) still cheaper then a VGA-Input Desktop Flat Panel.

I, too, have had this conundrum hit me. I also found out what xunker knows, that there are PCI-based cards to drive flatpanels. The most common use the same chipsets likely found in your laptop: Cirrus Logic, Chips & Technologies, or NeoMagic.

The one useful thing I have to add is that you must be careful about your power supply. The LCD panel requires power (not much of it, natch) but in some cases may expect to draw this power over the interface (there's a separate cable for it, but sometimes it's integrated into the connector). Well, if you have a hodge-podge PC built from older stuff with lots of slots filled (as many of us might, as that's just the sort of box that would be ideal to hide under the bed and put a small panel on) you may find that your bus doesn't have the power to run the panel. If this is the case, you'll either get a machine that refuses to boot, or one that shuts down unexpectedly - a lot.

The best thing to do is wire the power circuit for the flatpanel into another power source - either over the keyboard bus or from an external power supply. Careful, though; doing this running a cable into the PC and splicing into one of your power supply drive connectors will only work iff your flatpanel happens to need the same voltage - which is unlikely. Make sure you check.

For those who want to try this, the hi tech cafe (www.hitechcafe.com) and The Computer Geeks (www.compgeeks.com) sometimes liquidate spare parts including laptop displays...

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