TMDS interface design/hack

[gnif]

Hi All,

I recently came into posession of a stack of 19" LCD monitors that are in perfect condition, except they only have a VGA input on them. I want to set three of them up on my daughters computer, which only has DVI outputs, only one of which has analogue out. I have stripped one of the monitors down and found that the IC supports a DVI input, and identified the pins, but it is not wired up.

Comparing to the DVI input on another monitor, it has what looks like a FET on the DVI input and then is decoupled via a cap before entering the chip, what exactly is this doing? I understand the decoupling cap, but not the FET.

Also, I understand that the firmware may prevent it even using the input, but I have nothing to loose but some time giving this a go.

Edit: Here is another board using the same chip that I have used to identify the DVI pins: http://remont-aud.net/_bd/35/67957818.jpg, obviously this one is unpopulated. Mine does not even have the unpopulated circuit.

[Stonent]

Perhaps the FET is being used to allow for a different signaling current? So whatever the PC is outputting is not what the chip wants so the FET is being used to allow a different current through that's more to the chip's liking. Just a guess, I may be wrong.

[marshallh]

Probably ESD clamp diodes.. DVI input enable is probably set via pin strapping high/low or maybe setting in i2c/spi flash or eeprom.

[gnif]

Probably ESD clamp diodes.. DVI input enable is probably set via pin strapping high/low or maybe setting in i2c/spi flash or eeprom.

I think they might be actually now you mention it since I have seen some LCD interface boards that have the DVI input but leave these pads unpopulated. As for enable, yes, I hope it is something as simple as a strap, otherwise I will have to dump the eeprom and see what I can find.