Laptop TTL LCD cable extension

Dranger · 05-29-2006, 09:09 AM

Hello, my first post on these forums.

I would like to extend the cable between a laptop and it's LCD 12" panel, about 4 feet. This subject has be treated several times and there is a FAQ about it but something is not clear (at least for me

).

I have read several times here that extending a LVDS cable should be a breeze, but of course, my old laptop seems to have a TTL cable (P0~P23 pins, plus 6 others) and many people say TTL cable should not exceed 18 inchs.
At least the inverter is directly in the LCD panel case, so I won't have any problem with it.

On the other hand I can see at least two persons, Yuriy and 99Bravada who wrote they successfully extended a TTL cable to 4 and 6 feet without any interference, with a printer cable or an IDE *unshielded* cable.

I really wonder if I should try to extend my cable or not, and what cable I should use for that (simple flat IDE unshielded, or printer cable centronics 36 wires, or SCSI double shielded cable 50 wires)?

Thank you!

Dranger · 05-30-2006, 06:04 AM

I was thinking that if I use 800x600 resolution, then the signal frequency is 40 Mhz, and if I use 640x480, the frequency is 25 mhz.

The frequency of the signal is the same for VGA signal or TTL or LVDS...the only difference is how this signal is transmitted.
For VGA and LVDS, R G and B signals have different levels, LVDS uses twisted pairs and differential signal so it is very safe. VGA uses small coaxial wires, one per color.
For TTL, it is one wire per bit, on or off.

It seems to me that TTL should be even more resistant to noise and cable length problems than VGA because TTL signal is stronger and binary. If we have 1 or more meter VGA cables we should have 1 meter or more TTL cable?

SCSI III cables transmit a 40 Mhz signal (exactly the frequency required by 800x600 SVGA) using 34 shielded twisted pairs.
Using such cable, it is even possible to put each TTL wire in a separate shielded twisted pair.
I am not used at all to cables and signals.... do you see a reason why extending TTL LCD cable this way should not work?

davesaudio2 · 05-31-2006, 08:03 PM

in the context of LCD panels LVDS means serialized/deserialized as well as
differential signalling

Dranger · 06-01-2006, 03:54 AM

ah ok, thank you. So LVDS signal has a higher frequency than VGA and TTL signals.

I think that when it is written that 40 Mhz TTL signal should not travel more than 18 inchs in a cable, it does not concern display signal but data signal like in a bus.
Using a 3 or 6 feet cable, properly shielded, for TTL LCD signal will definitely introduce some noise and distorsion.
The noise will not affect too much the picture because it is refreshed 60 times per second.

The distorsion may cause the LCD controller not to recognize properly Vsync, Hsync and neighbor pixels can be mixed together too. This is the bigger problem.

According to the two persons here who successfully extended TTL LCD cables over 3 and 6 feet, it surely works in some cases. Maybe they have LCD controllers which are very good and cope with noise and distorsion....or maybe it means that 3 or 6 feets TTL LCD extension still gives a signal good enough for a LCD controller.

It would be very nice if some persons could share their experience on this subject.

Dranger · 06-17-2006, 05:21 PM

OK, I extended the cable and it works perfectly.

I used an old SCSI cable with 50 wires inside...not even twisted pairs...
Everything is going through this cable...DC power for inverter, leds, laptop power button, and 30 wires for LCD signal and sync.

I tried to do it as clean as possible...everything is soldered, shielded, and shield is connected to laptop frame.

The cable is 1.80 meters long (6 feet)
No artefact, no distorsion in 800x600 24bits...no brightness lost...the picture is unchanged. Some distorsion appears when I use 32 bits colors....but anyway this LCD is unable to display 32 bits colors properly...so it does not matter.

I confirm it is possible to extend old laptops screen cable over 6 feet, TTL or not, without problem if you don't use crazy resolution...800x600x24bits works perfectly.

eprench · 07-01-2006, 02:27 PM

Hey man, Im thinking about installing an old Compaq Presario 1250 and I am glad I found your post. Is there any chance you can email me some pictures to see how you shielded and got everything all clean and nice?

Thanks a lot!

eRnie P.
VLD, GA

Dranger · 07-03-2006, 02:35 AM

In fact I didn't take any picture. I used an old SCSI cable with 50 wires I got for 5 euros...the wires inside are not even twisted pairs.

I just open the cable both sides on 15 cms, then I cut and remove the shield, but I keep the wire which is going through the shield.

Then, to avoid any mistake with colors, I connected one wire at a time: I choose one of the wires between laptop and LCD, I cut it, I solder to the cable, I check connection. Boring.

Each connection was covered with...don't know how it is in english...shrinking sheath?

Then I covered everything with aluminium foil, and I soldered the shield wire on the laptop frame and on the other side on the LCD frame. Then I used black tape.
I am not very satisfied about the black tape...it does not look nice because the cable does not have a constant size (43 connections soldered...it's big). But it is hidden by the screen anyway.

To start the laptop, I use a microswitch added on the LCD screen. Wires for this microswitch go through the SCSI cable too.
Last problem...I will have to paint the cable black, to make it less visible...it is not hidden everywhere in the car.

I could not integrate this LCD (12", far too large), it is just put on the gear box...but I was lucky, this place is big enough, well protected from direct sun light, and Laptop unit is under passenger seat, I use a radio keyboard with joystick-mouse to control it.

This small install needed an overall investment of ... 50 euros (mostly power supply and keyboard)

eprench · 07-03-2006, 10:56 AM

So, ok, you got the monitor going and working, so... where to you stick a massive 12" monitor? I drive a wee escort and have like NO SPACE for it... any suggestions?

Dranger · 07-05-2006, 11:32 AM

In my car it fits between the bottom of the board and the gear box....there is a large empty space there with just an ash tray and I don't smoke in car.

Sure the screen is quite large, but if you find an acceptable place for it, it is 10 times better than PDA and 7" screens for GPS. If needed, map is readable at once, as easily as speed meter. This is the cheapest install...for more integration, laptop+VGA 7" screen for car.

eprench · 07-05-2006, 11:36 AM

Ok, I've been thinking about my glove compartment... I'll post some pix when it is ready.

Friedrice · 08-21-2006, 08:34 AM

Anyone have any pics of a laptop screen install?

eprench · 08-21-2006, 03:01 PM

The screen is totally doable, but too big for my escort... Im waiting untill I can afford a truck to install the massive screen. Meanwhile I guess a 7" will have 2 do.

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