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Connecting a 4 channels LVDS LCD (without EDID) to a 3 channels motherboard

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NiHaoMike:
Might it be simpler to just use the original driver board to connect it to DVI/HDMI/Displayport?

hlavac:
Maybe try to duplicate the least significant bit into the extra lsbs or you may lose some dynamic range...

c64:

--- Quote from: PaddleStroke on August 02, 2020, 01:39:27 pm ---In the 20" panel datasheet, they say "Interface chip must be used LVDS, part No. DS90CF383MTD(Transmitter) made by National Semiconductor.
Or used the compatible interface chips(TI:SN75LVDS83)."

It's not exactly the same P/N as you said, but it's probably very similar chips I guess? Though you also advised the SN75LVDS83 too which is said to be compatible in the datasheet.

--- End quote ---

Should work, the interface is standard.


--- Quote from: PaddleStroke on August 02, 2020, 01:39:27 pm ---So if I'm guessing correctly, it just needs to connect as follow :
Host LVDS connector -> receiver -> RGB18 bits -> transmitter -> panel LVDS connector.

It only needs to connect correctly the RGB output from the receiver to the input of transmitter while leaving LSB to GND, right? Following the mappings given in the panels datasheets ? Now the 20" panel datasheet does not give the mapping of the bits inside the channels. But looking at DS90CF383MTD datasheet at figure 2, I think that's the mapping to follow isn't it?

--- End quote ---
There is no "standard mapping", lvds83 datasheet describes some most commonly used ones. If the panel datasheet doesn't provide mapping, you can only guess which one they used. Or if you have fpga development board, you can try to drive your monitor, to find out the mapping.

Berni:
Often the 4 lane LVDS mapping is made in such a way that the first 3 lanes carry the upper 6bits of each color + sync signals. Then the 4th lane just carries the extra 2 bits of each color. In those cases the display will run just fine with the 4th pair not used, it will just only have 18bit colors.

Last time i had to identify LVDS lines (It was a scope motherboard with a LVDS display and i needed the pinout) i simply poked the LVDS lines with a scope and looked at what they are doing. Various colors can be put on the screen to find where the color bits are. Tho you do need a fairly fast scope since the bitrates are very high.

The EDID is indeed just a I2C EEPROM on the lines, you can find the data structure that needs to be written into them on the internet, since you have a different resolution on the new display means you have to modify the data to report the correct thing. However there is no grantee that the motherboards BIOS actually reads the EDID. On boot the BIOS will set up the graphics card to output the correct LVDS format and in motherboards that are only ever supposed to have one display connected this is sometimes hardcoded in. Or perhaps it might use the EDID to determine from say a list of 4 different displays that this laptop model comes with and choose the right hardcoded initialization routine for it. So check that you can actually convince your motherboard into outputting the correct resolution first.

If not then you can still tap into something like a HDMI port where EDID is always used so you can make it give you any resolution you want. But that involves HDMI to LVDS conversion too.

PaddleStroke:
Thank you all for your replies and insights!


--- Quote from: Berni on August 03, 2020, 05:49:21 am ---However there is no grantee that the motherboards BIOS actually reads the EDID.

--- End quote ---
You raise a very interesting question concerning the bios. I kind of fear going all the way then realizing that the bios only supports 1600*900. As 1600*1200 is not the same aspect ratio there is no reason the manufacturer would include it. Though it sounds like the bios could support any resolution without trouble and it would be rather the bios excluding voluntarily other resolutions.

To test that I could try to write a EDID for another resolution and load this EDID to the current panel. Then see if the display shows garbage or a somewhat truncated image. As I can use an external monitor to setup back the original EDID I guess.


--- Quote from: Berni on August 03, 2020, 05:49:21 am ---Often the 4 lane LVDS mapping is made in such a way that the first 3 lanes carry the upper 6bits of each color + sync signals. Then the 4th lane just carries the extra 2 bits of each color. In those cases the display will run just fine with the 4th pair not used, it will just only have 18bit colors.

--- End quote ---
What you say makes sense, it's what I was expecting. But the thing is the 20" datasheet does not provide any mapping for the LVDS signal. It only says "Interface chip must be used LVDS, part No. DS90CF383MTD ..." Then looking at this DS90CF383MTD datasheet there seem to be an offered mapping in figure 2, which is not 4th lane with extra 2 bits of each color, and I don't know if that is what they used anyway.

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