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Sorry, new here, first build.

Can't seem to get my risers to fit in the larger PCI slots...

http://i65.tinypic.com/fona5f.jpg

Also, do I need a connector between the two PSUs like the ones sold by add2psu?

1x PCI-E risers will only work on PCI-E slots. PCI slots use a diffrent voltage than PCI-E. If you connect a PCI-E riser to a PCI slot, bad things will happen. Not limited to smoke, sparks and fire. In a multi-psu setup it's important the PSU's share the same ground. The Add2PSU connector is an easy way to do that and it makes the PSU's come on and shutoff together. Using an ATX 24-pin splitter cable is another way.

https://www.amazon.com/Female-Power-Supply-Motherboard-Adapter/dp/B013HTWI9G

You also should only use the second PSU for the VGA power inputs. The main PSU connected to the motherboard should also power all the risers and accessories connected to the motherboard to avoid ground loops.

https://i.imgur.com/BCnc7Uh.jpg

https://bitcointalk.org/index.php?topic=443540.msg4878774#msg4878774

What motherboard do you use?  As first responder already told: a PCI slot is not suitable for PCI-E Risers. PCI-E risers must be put in PCI-E slots
A good mining motherboard would be an Asus  Z270-A It has 7 PCI-E slots available.

Thanks for the help! Yea, old motherboard only has 4 pcie slots, so just ordered a new one that will support 6.

actually both ways of connecting two PSUs are correct if you are using "USB" risers

As it says in the description of the Thermaltake ATX splitter cable, who also makes PSU's.

https://www.amazon.com/Thermaltake-24-Pin-Mining-Adapter-AC-005-CNONAN-P1/dp/B00O0M6Q0C


You should never mix power supplies to devices connected to the motherboard. USB risers are not electrically isolated from the motherboard and therefore it's important that all the risers are powered by the same PSU that powers the motherboard.

I also noticed BBT in some of his his latest live streams emphatically recommends that for a dual PSU rig, you need to power the risers with the same PSU that powers the motherboard and only use the second PSU to power the GPU's.

https://www.youtube.com/watch?v=lXZondkAWVc&feature=youtu.be&t=1253

https://youtu.be/pQ-EAunoAqY?t=988

It's true that you should never mix connections to the motherboard, but that's not at all what is happening with USB risers.  USB risers do not send power to the motherboard - they send it to the video card.  The only thing transferred between the riser and the motherboard is data.  Yes, it's using electricity to do that, but at very low levels - for enough power to be sent over that connection to cause damage to hardware, something else would have had to catastrophically fail.

Mixing PSUs in general is a really, really bad idea, but if you have to do it, each GPU + Riser should be considered a whole unit.  Voltage imbalance between two sources can fry whatever is in between.  The video card is what is in between the sources, not the motherboard.

I tested the outer pins of the USB cable with a multimeter. There was 3.3V coming from the motherboard to the the connector on the riser and there is also a ground connection between the riser and the motherboard. There is no problem with mixing power supplies if done correctly.  As explained by BBT and the link I posted in my original post, the problem is not the amount of current, it's mixing grounds from multiple sources. Which is why you MUST power all the risers with the same PSU that powers the motherboard to avoid ground loops.

actually grounds from multiple sources SHOULD be connected to have the same potential among power sources.

powering USB risers from secondary PSU is OK and works on my farm

I would bet BBT has built more rigs than most here. I would take his advice and that of a PSU manufacturer that you should never mix power connections to the motherboard, which is exactly what's happening when you power a riser with another PSU that's not connected to the motherboard.

you were told already, when using USB risers you do not send any power to motherboard, dunno what it is so hard for you to comprehend...

btw that BBT idiot claimed sgminer in linux do not have RX hashrate drop, guess what? it have...

I comprehend that you don't understand USB risers are not electrically isolated from the motherboard and share a 3.3V AND ground connection to the motherboard. You also don't understand what BBT said and is doing. He reached out to AMD and others to see if an issue that affects most miners can be addressed. What BBT claimed and showed was that when he reached out to AMD to discuss the issue about the Polaris line and the Claymore epoch benchmarks figures showing a significant hash drop in future epoch's, in response he received a confirmation from a senior manager at AMD that large professional miners were using SGminer in Linux with OpenCL 2.0 and DO NOT have the same issue, which is what he reported and showed. He also asked the community to verify those claims to make sure they were correct. After receiving feedback from members here and others stating SGMiner seems to be affected as well, he has reached out to AMD again asking for clarification to their original statement. Once he receives a response, he will let everyone know.

https://www.youtube.com/watch?v=l3XOtQtFKSQ&feature=youtu.be&t=978

https://youtu.be/ZzAbECBnJmY?t=4031

https://www.youtube.com/watch?v=PXoPFPqU3Y8&feature=youtu.be&t=4822

I for one, applaud BBT for looking out for us and hopefully getting this issue resolved as I'm sure many others do as well.

That is not the case. USB risers do not provide any voltage from motherboard except contol signals. Thus they are isolated.
And you are lying, they are not sharing 3.3V. They do share ground however as all devices should.

You can easily verify yourself, as I did with a multimeter, the outer pins of the USB cable connected to the motherboard have 3.3V coming from the motherboard connecting to the riser. Your confirmation that they share a ground and yet are electrically isolated shows you are clueless and I would DEFINITELY NOT listen to your advice.

that is exactly what i did - of course there is no 3.3V connected.
actually there is no logic connecting it as riser makes it's own 3.3V from 5V input...


sharing ground does not mean that devices can not be electrically isolated.
your TV and microwave owen share ground yet they are electrically isolated.

you are uneducated idiot no wonder you gather your "knowledge" from other idiots on youtube lol

I proved and anyone that knows how to use a multimeter can prove as well the riser and motherboard ARE NOT electrically isolated and therfore as recommended by a PSU manufacturer, and someone with over 15,000 people following him on YouTube for his experience with mining rigs, you should never mix power supplies to devices connected to the motherboard. The post below from someone with an electronics backgorund also explains why the same PSU that powers the motherboard MUST also power the risers.

https://bitcointalk.org/index.php?topic=1843586.msg18399752#msg18399752

Which is a lot more reason to follow that advice than from someone too stupid to use a multimeter and understand again, that risers ARE NOT electrically isolated from the motherboard.

OK I'm done arguing with idiots lol

The fact that you have to resort to name calling and slander to unsuccessfully TRY and prove your point shows you have ZERO credibility and only an idiot like yourself would listen to you.

Idiots always thinks it's name calling while in reality it is stating a fact. You are a fool who can not process information properly.
Thus following other fools on YouTube.

Says the idiot that proves nothing and simply repeats his ignorant and easily demonstrably false claims. LOL

Well that escalated quickly...

I have 10 rigs for more than 1 year, some of them are mixing with 3 psus, never had any problems that is power related.

Ps: im using paper clips jumper in all Psus that are not powering motherboard

The rest are just bullshit to sell some gimnicks acessories/psus

#peace

So you're not synchronizing PSUs when powering-on/off? I'm just wondering because I don't do that either and I have up to 4 (four) PSU's on some of my rigs.

One day I will probably measure currents flowing between PCI-e and the risers, but I don't think there is much if anything at all. 3.3V seen on the rizer USB connector is likely signals, not VDD.

PCI-E lanes that carry data are usually AC coupled via small capacitors on all GPUs. They are not isolated (like optically), but not DC coupled either.

-scsi

Just checked some risers i have laying around (v006 and v006c) and they don't conduct neither 3.3VDC nor 12VDC power from the motherboard to the 16x slot on the riser. Though your mileage may vary...

All ground lines on my risers are connected through to the 16x slot as they should, therefore powering risers/GPUs from whatever PSU should be acceptable as long as all grounds are reliably connected directly or otherwise via USB3.0 cables.

This ground connection does not have to carry high currents as all power hungry devices, such as GPUs, get their own high capacity GND and 12VDC lines from respective power sources. In this way, the common GND connectivity throughout the system is supposed to carry small currents necessary to equalize GND levels from different PSUs.

This of course is an ideal situation assuming quality PSUs and risers being used without any miswiring or shorts. In real world anything can happen, but I can imagine worse outcomes possible in the most commonly recommended wiring scenario with all risers being powered from the main PSU and some GPUs powered from a slave PSU(s).

In such situation those GPUs will receive one chunk of their 12VDC power (up to 6.25A or 75W) from main PSU via riser and the rest (100W?) from the slave PSU via PCIe 8-pin connector. These 12V levels can not be exactly the same because they are coming from different PSUs. You can imagine some horribly designed GPU board where these different 12V buses meet somewhere on the board and then smoke comes out.

In my opinion, the most important thing to understand here is that feeding each particular power hungry device (GPU) from the same source is much safer, therefore they need to have their 12VDC lines via PCIe connector (from the riser) and via 8-pin PCIe power connector to be coming from the same PSU.

One other common misconception I see being thrown around is "Ground Loops" and the myth which says that you somehow can avoid it. These things are impossible to avoid in dual PSU configuration simply because GND lines do connect inside each GPU. Even if we feed power to them from different sources (riser and 8-pin), grounds from those sources still meet on the ground plane on the GPU board. They also meet on the motherboard, when more than one GPU is involved and in many other situations.

In general, ground loops are bad, but in our situation with PCI-Express interconnect, we can mostly ignore them because the bus uses differential signaling which is largely insensitive to the noise and garbage caused by ground loops.

-scsi

brainless idiots won't listen, they have "guru" on youtube with 15K followers (lol) telling them to power risers only from main PSU