eGPU – Connecting an external video card to a notebook (DIY implementation)
Due to constraints like orientation, position of the slots on the expansion board and the size of the video card, and trying to fit everything inside some sort of a case, I chose to use a PCI Express riser.
A couple of things about the PCI Express socket. There are many pins in that socket, and some of them are for transmitting data while others are for providing 12V and 3.3V.
A Nvidia GTX 680 card is power hungry card and it will exceed the max of 25W provided by the original Sonnet Echo bundled 12V power brick adapter. We also need some 3.3V lines to power it. The easiest solution for this is to use an ATX Power Supply because that one can provide both required voltages and also enough power for any highend video card.
The riser extender cable was marked for both sides, required wires were identified and then cut and soldered to an ATX motherboard extender cable in order to be powered from the PSU.
The operation above was also required in order to avoid some issues (PERST# delay) with computer having time to „see” the video card after doing POST initializations by the BIOS. You may ask why the 12V wires were splitted, considering the card could draw the voltages over the dual 8 pins connectors from the ATX PSU. Well, the Sonnet Echo Expansion board was also powered from 12V from the original brick adapter, and allowing current to flow from both ways might not be a clever ideea.
Also, why did I cut the first 3 wires for 12V, when the PCI Express specification labels the third pin as Reserved and only the first two pins are for 12V? The reason is simple, please get a video card and take a look at the pins, you will see that first three pins on side B are connected and like mentioned above you don’t want current coming from ATX PSU to flow back into the Sonnet Echo Expansion board.
After connecting all pieces together, we are ready to power on the whole thing, and it works, at least at the electrical level.