I managed to break the PCIE_PEG_TXN 2 pin on my motherboard according to this pin layout: https://sector.biz.ua/docs/lga-775-1150-1151-1156-1155-1366-2011-pinouts/s1200_1.png I am curious if it is wise to start the computer without this pin. I assume it is for sending data to the first PCIE slot on the second PCIE lane. The positive is still there, but I guess without this negative pin it is not possible to close the circuit. Maybe it is not a big deal and I will just lose a PCIE lane, maybe it is a big issue and the CPU, motherboard, video card will burn down and it will cause meltdown in the near nuclear reactor. What do you think?
I'll assume by "broken" you mean physically snapped the pin off (i.e. didn't short it out etc.).
In this case theoretically nothing catastrophic will happen. It is reasonably safe to boot the computer in such a state and for the most part it will work ok.
What you will find however is that your graphics card will not link up at the full x16 width (or if bifurcation is being used the first port won't link at x8). This is because without both the
_P and the
_N part of a lane, the lane will be dead.
Fortunately, the PCIe spec mandates that all PCIe devices must be able to support x1 mode. This means your graphics card will in all likelihood work just fine. During the link training process it will detect that one of the lanes is non-functional, and so fail back to x1 mode (*). Thus it will still work, just slower.
Some PCIe devices also support intermediate widths. If supported your graphics card may actually link up at x2 width, but there is no requirement to support that, so in all likelihood it will remain at x1 width. It will not link at a partial width (e.g. x15, or x8) because all lanes must be consecutive starting with Lane 0.
(*) If it had been Lane 0 that you broke, no link could be established as that is the one lane required to establish a link.
The TXP and TXN signals form a differential pair. Disconnecting one of the two will not by itself cause any damage, but obviously you wouldn't expect the affected lane to function.
If you're sure this is the only damaged pin, it should be perfectly safe to power up the system and check if the PEG slot still functions. It should even still work in a degraded mode (x8 instead of x16).