Splitting PSUs between 2 PCs

[danalog]

Strange question here but I want to test two HD 5870s in Xfire. Each needs two 6 pins. I would rather use two power supplies than split their 8 pins into 6 pins, seems safer (best PSU I have has two 8pins)

Would it be safe to have one card running off of the computer's PSU as normal but the other one from the PSU in a different computer? As in, the 24 pin would be plugged into that other computer (and running) but the 6 pins are to the GPU in a different system? Also seems safer than jumpstarting the PSU or shorting it with a paperclip.

Thanks!

[IkeaGnome]

Paper clip isn’t all that unsafe. If you want something shielded, it’s only 5V signal to turn the PSU on, I used one of the wires from some CAT5 for a while until I got my jumper. 
 

I don’t THINK any harm would come from the PSU running a different system, the problem would be with the GPU turning on. At least with what I had set up in my folding machine, the second PSU had to turn on at the correct time to get everything to initialize at once or there was no boot. 

[RollinLower]

7 minutes ago, IkeaGnome said:

the second PSU had to turn on at the correct time to get everything to initialize at once or there was no boot.

this is the main issue here, you need the two PSU's to turn on at the same time. normally you would do this with a splitter of some kind, but i'm not sure how to do it if the PSU installed in a different system.

[danalog]

6 minutes ago, RollinLower said:

this is the main issue here, you need the two PSU's to turn on at the same time. normally you would do this with a splitter of some kind, but i'm not sure how to do it if the PSU installed in a different system.

I'm thinking just press both power buttons at the same time. 

[Moonzy]

Don't need to bother with a second system

Just jump the ps_on wire to ground, then flip the PSU switch when you start up the first system

 

Or you could buy an adapter to do that automatically

 

Edit: something like this

[danalog]

5 minutes ago, Moonzy said:

Don't need to bother with a second system

Just jump the ps_on wire to ground, then flip the PSU switch when you start up the first system

 

Or you could buy an adapter to do that automatically

 

Edit: something like this

Thanks, good to know

 

I just thought. A PSU I bought years ago came with this little adapter for testing to make sure it turned on, as the PSU had a light on it and in the instructions it said to test for the light to come on before using it in a system. It goes on the 24 pin connector and has two pins shorted - I recall it being around pin 6 on the top and pin 7 on the bottom, they were one off IIRC. 

Is that basically the same as shorting it with a paperclip? I assume that's the same pins. Could just use that

[BondiBlue]

29 minutes ago, Mel0nMan said:

Also seems safer than jumpstarting the PSU or shorting it with a paperclip.

Shorting it with a paperclip would work perfectly fine, and you wouldn't have the extra complexity of a second system running. You wouldn't want that computer to accidentally turn off since one of the GPUs would lose power. 

[IkeaGnome]

22 minutes ago, RollinLower said:

this is the main issue here, you need the two PSU's to turn on at the same time. normally you would do this with a splitter of some kind, but i'm not sure how to do it if the PSU installed in a different system.

With the right timing, I'd imagine it's possible. There's much easier ways though.

9 minutes ago, Mel0nMan said:

It goes on the 24 pin connector and has two pins shorted - I recall it being around pin 6 on the top and pin 7 on the bottom, they were one off IIRC. 

Should be pins 4 and 5 of the top row, or 16 and 17 of the connector. The PSU_ON to any ground will work though.

A cable is half the price and much more simple than the adapter above btw.

https://www.amazon.com/Supply-Extension-Motherboard-Chassis-line，24/dp/B091G9VZ5P/ref=sr_1_3?keywords=dual+psu+adapter&qid=1648735765&sprefix=dual+psu+%2Caps%2C156&sr=8-3

 

[Moonzy]

22 minutes ago, IkeaGnome said:

A cable is half the price and much more simple than the adapter above btw.

https://www.amazon.com/Supply-Extension-Motherboard-Chassis-line，24/dp/B091G9VZ5P/ref=sr_1_3?keywords=dual+psu+adapter&qid=1648735765&sprefix=dual+psu+%2Caps%2C156&sr=8-3

Using a relay would be better electrically, since you're isolating the circuits from each other on the +ve side

Plus if you're gonna run dual GPU setup, it's gonna be some load on the pcie slot power, have to take that into consideration too

[mariushm]

You'd want to remove the PS_ON wire from the second computer's 24 pin header... only one computer should control the power supply or neither of them.  

 

Only one motherboard should issue the signal to the power supply to turn on. 

 

The absolute minimums are

1. the PS_OK wire which tells the chipset that the power supply is ready to provide all voltages (basically turn fully on)  and

2. the 5v stand-by wire (purple) (because 5v stand-by is powering the chipset which "listens" for your on/off button press and checks that power supply is ready by looking at PS_ON signal) 

3. at least one ground wire (COM) as return for 5v stand-by and to have both computers at same reference level. 

 

Easiest would be to get a temporary push button  with TWO pairs of pins , one pair for each  PW_SW in the front panel header of each motherboard.  When you press the button, both motherboards chipsets' detect the button pressed and send the signal to the power supply to turn on, but only the main motherboard will actually hold down the PS_ON to ground (like you would short the PS_ON with a paperclip to ground). 

 

This way you guarantee that both computers receive power from power supply at same time and only AFTER that the the chipsets and the BIOS start initializing hardware. 

 

Note that this way, you can't use the button to "sleep"  (short press).

if you'll press your new power button, a short press,  BOTH computers will launch the "go to sleep" and attempt to go to sleep, but if the main one finishes before the second one, it will cut power off to the power supply (because main pc controls the PS_ON) and the second pc will be pissed (like power failure as it suspends stuff to disk / goes to sleep, unclean go to sleep).

Same if you shut down the first pc, the first pc will shut down the PC and turn off the second. 

 

But if you shut down the second PC , THEN shut down the first all is good.

 

The splitters from companies like Phantek or whatever have a microcontroller or some chip which do the PS_ON to ground instead of letting one of the motherboards turn off both computers.