Question EPS12V, Rosewill Capstone PSU & ASUS TUF Gaming X570-PLUS

[Landstander]

My previous PC build failed after 5 years. The motherboard was a M5A99X EVO R2.0 (https://www.asus.com/Motherboards/M5A99X_EVO_R20/) and the PSU was a Rosewill Capstone-650-M.
So I'm looking at the ASUS TUF Gaming X570-PLUS board and I've got a sinking feeling that not only is my PSU wrong... it may have contributed to my last system's failure.

My previous board had a power connect labeled EPU with 8-pins. It looked like the EATX12V_1 in this photo from the TUF Gaming board :

My first question is that the rounded and square sockets are NOT identical the 4+4 pins on my Rosewil, but it seated okay and I didn't notice the discrepancy while I was building it. The plug looks like this:

If you can't make it out, from top left to bottom right: Round, Square, Round, Round and Square, Round, Round, Round.
So some round plugs are going into square sockets.

Now I've seen forum discussions and build videos say the 4+4 EPS12V is the same as an 8-pin when installing it, but it obviously isn't. Did I screw up or are they meant to be compatible?

The second question is about the wording in the TUF Gaming manual. It says:

- DO NOT connect the 4-pin power plug only, the motherboard may overheat under heavy usage.
- Ensure to connect the 8-pin power plug, or both the 8-pin and 4-pin power plugs.

Was I wrong before? Was I right?
If the 4+4 IS correctly a substitute for the 8-pin, what happens on the TUF Gaming with ONLY the 8-pin and no 4-pin connected?
Is there some deficit, stability problem or overclocking issue?

 

[Landstander]

Let me answer my own question for posterity:
According to Playtool.com (http://www.playtool.com/pages/psuconnectors/connectors.html#eps4plus4):

If you look carefully at the image above then you can see the polarization of the pins which prevents you from plugging the cable in improperly. Some of the pins are square and some of them have rounded off corners. The motherboard connectors have matching square and rounded off corners to prevent the cable from being plugged in the wrong way. But if you look really carefully at the right half of this particular cable and then look at the 8 pin 12 volt cable pictured above you'll notice that they don't match. A regular 8 pin cable has four square pins and four rounded ones but the 4+4 cable shown above has two square pins and 6 rounded ones. The left half of the 4+4 matches the left half of an 8 pin cable but the right half is different. Hmmmm... And this isn't some bizarre cable either. I've seen plenty of 4+4s which look like this one. And then there are other 4+4 cables which look just like an 8 pin cable split in two (which makes sense). Since rounded pins fit into square holes in motherboard connectors, this particular cable will fit just fine into an 8 pin 12 volt motherboard connector. But both halves of this 4+4 will fit into a 4 pin 12 volt motherboard connector. You're supposed to use the left half of the cable shown above when plugging it into a 4 pin motherboard connector but the right half will also fit. As it happens, either half will work fine in a 4 pin motherboard because both halves of the 4+4 just provide 12 volts. The pinouts are the same for both halves so either one will work. I'm not sure why they make cables like this one because you'd figure a 4+4 cable would just be an 8 pin cable which splits in two. And you only need one half of a 4+4 cable to plug into a 4 pin motherboard. The other half is unused. But the kind of 4+4 cable shown above is pretty common so don't let it throw you.

So, yes, the 4+4 in the 8 pin is right.

AND according to Asus support, plugging in ONLY the 8-pin EPS12V will under power the board, BUT the support guy ALSO claimed using an adapter (off a PCIe cable?) molex will fulfill that requirement.

So assuming the latter is true, everything is good.

Edit: What bugs me is I'm sure I've got one of those cables around somewhere from back when they first added the ATX 4-pin. I'll probably still get a new one. https://www.amazon.com/Monoprice-101321-Power-Supply-Adaptor/dp/B005GBILYU/