Hello everyone. I’m working on something and have hit a snag and could use your help to resolve it. I’m trying to help a friend with a project where he is modifying an LCD monitor and a graphics tablet for a PC. The end result of the modification is that he will be able to draw driectly onto the monitor instead of having to draw onto the graphics tablet. I’m pretty sure the tutorial he is following is here:

Bongofish - DIY Cintiq - Part 1

As part of the modification, he has taken apart the LCD monitor. Inside the monitor is of course the LCD itself and associated lighting but also inside are two PCBs connected to the back shell. One PCB is the power supply/inverter board and the other one has the LCD driver ICs.

The power supply/inverter board connects to the CFL lights on the edges of the LCD through a pair of wires. There are two edge lights so 4 wires total.

The other board connects to the LCD through a 34 pin data cable. Here is a picture of the cable:

http://www.electro-tech-online.com/a...r-imag0068.jpg

Since this monitor was never intended to be taken apart, the cables inside (data cable, CFL cables) are much too short for his purposes so he asked me to lengthen themfrom about 6 inches to about 5 feet.

I was able to make an extention cable for the data cable using some 2mm IDC connectors, a 2mm header and some 1mm pitch ribbon cable. I did not put any kind of suppresor around the cable. Should I have? For the lights, I just spliced and soldered extra wire to the CFL wires to lengthen them. Sounds simple right? Well, neither one of the “extension” cables works as expected.

First off, the data cable does work well enough to display an image but it is not the complete image. It is distorted and there are some horizontal lines that obscure the image. If I had to guess I’d say it seems like there is some interfence or problem with the clock signal that’s causing certain pixels to show up in the wrong place on the monitor.

Second, the CFL lights don’t light up all the way. It appears they only light up about halfway down the length of the tube. The half that is lit is still dim compared to what it should be and the other half isn’t lit at all. I would have expected some line loss with the extra cable length but this seems excessive.

My questions:

What have I neglected to consider?

Short of returning the cables to their original lengths what should I do to make the extension cable work both the CFL and data cables?

Please let me know if any of my descriptions are unclear or if you need some more information. Any insight or ideas would be much appreciated. Thanks in advance.

I am bloody amazed that you get any data trough such a parallel data cable over these kind of distance.

even with a conservative 50Hz, 3*1024*768*8 we are talking about 1 Gb/s. Supposing that the cable is not differential pairs but simple ground we are talking 250Mhz. 500 if it is differential pairs. The driver is meant to deal with 15cm of cable, you multiply the capacitance of it by roughly 10. As I understant you removed the ferrites around the cable, so you are receiving high frequency garbage all over.

As for the Psu cable, if I remember right we are talking a few kV for CFL, so I guess the insulation might not be quite up to the job if it is paired cable.

Superfrog,

Thanks for the input. I want to make sure I understand your explanation correctly.

Originally Posted by superfrog

even with a conservative 50Hz, 3*1024*768*8 we are talking about 1 Gb/s. Supposing that the cable is not differential pairs but simple ground we are talking 250Mhz. 500 if it is differential pairs. The driver is meant to deal with 15cm of cable, you multiply the capacitance of it by roughly 10.

I'm a physicist, not a EE so bear with me. The numbers 3*1024*768*8 come from:

3 - the 3 colors, RGB
1024*768 - the total number of pixels
8 - the level of each color (0 - 255)

Therefore, 3*1024*768*8*50Hz = 943,718,400 bits/s ≈ 1 Gb/s

Is this correct?

Even if my derivation is correct I still don't understand why the driver board would be operating at 1 Gb/s. Wouldn't the 1 Gb/s only be necessary if you were pumping all those bits through one wire? That's kind of a side point that I just want to make sure I understand.

That aside, I think what your saying is that the increased wire length significantly increased the inherent capacitance of the wire and now the bits aren't rising and falling fast enough because the driver board was designed to output through a wire with a capacitance roughly equal to what a 15cm wire would have. Is this a correct interpretation?

If this is correct, other than redesigning the driver board is there a way to work around this problem?

Originally Posted by superfrog

As I understand you removed the ferrites around the cable, so you are receiving high frequency garbage all over.

This is not correct. The original cable in the photograph from my first post is still part of the circuit. I plugged the extension cable into the driver board and then connected the other end of the extension cable to the original cable. I connected the female IDC on the extension cable to the female on the original cable using a box header. I then plug the free end of the original cable (the lighter colored connector in the photograph) into the LCD. I left the ferrite suppressor on the original cable in place but I didn't add any new suppressors to the 5 ft extension cable. Should I have? Would it make a difference?

Originally Posted by superfrog

As for the Psu cable, if I remember right we are talking a few kV for CFL, so I guess the insulation might not be quite up to the job if it is paired cable.

It is a paired cable. I'm not sure how much voltage the inverter should be producing. I did measure the voltage on the PCB after I lengthened the cables and started getting the lighting problem and it was ≈ 350 VAC. Can you elaborate a little on how the insulation of the wire is affecting the voltage? Should I use a larger gauge wire or a thicker insulator? What can I do to mitigate this issue?


Thanks for your help.