Mixing PCIe 4.0 and 3.0 GPUs for multi-monitor setup

[urbanhusky]

Hi,

 

I'm going to upgrade to a 6600 XT or similar and I'm using 5 monitors. I want to use my old RX 480 to drive a few monitors I use for Desktop applications (Discord, Firefox, Ableton, ...) because the 6600 XT only has 4 outputs.

I'll have to get a new motherboard so that I can fit everything (2 GPUs and 2+ USB controller cards - no hubs are not a viable option) so I might as well upgrade from my B450 and gain PCIe 4.0.

 

It is my understanding that the two GPUs would be restricted to 8x lanes each, but will the 3.0 card impact the 4.0 card, i.e. can those lanes run on separate clocks/PCIe-gen, or will the 6600 XT drop down to 8 lanes with PCIe 3.0 speed?
I would underclock the RX 480 because I don't need gaming performance from it and I hope to be fine with my 650W PSU driving the entire PC (3700X, 64GB DDR4 3200, M.2, SSD, HDD, optical)

Are there any other issues I should expect?

[mariushm]

Uhmm.. no.

 

It depends on motherboard if you can split the pci-e x16 slot for the video card into 2 pci-e x8 slots.

It's not possible on B450 motherboards, it should be possible on x470 motherboards (depends on motherboard maker), it should be possible on x570 boards (again, if motherboard maker does it, with SLI and Crossfire dead, some don't bother).

 

As far as I know B550 chipset also doesn't allow  splitting the 16 lanes into 2 x8 slots, but motherboard maker may choose to add an extra chip to split the lanes - for example a Gigabyte B550 Aorus Pro something (or Master, I don't remember) was designed to split the x16 into pci-e x8 for video cards and two M.2 connectors for SSDs.

 

On B450 chipset boards you have 24 pci-e lanes from cpu  and 8 pci-e 2.0 lanes from chipset. B550 also has 8 pci-e lanes, and x570 has 10 pci-e lanes if my memory is correct.

 

24 pci-e lanes from cpu are split into

 4 for the m.2 connector

 4 for the chipset connection

 16 for the video card

 

The chipset pci-e lanes are usually routed to a M.2 connector and the bottom pci-e x16 slot, which has only 4 electrical pci-e lanes.

 

You should put the RX480 on the most bottom pci-e x16 slot, with pci-e lanes from chipset.

 

You could even use a pci-e riser cable to use a pci-e x1 slot that's often blocked by video cards otherwise and unused.

[urbanhusky]

54 minutes ago, mariushm said:

It depends on motherboard if you can split the pci-e x16 slot for the video card into 2 pci-e x8 slots.

My intention isn't to split a single slot, but rather use two mechanical x16 slots on the motherboard. I worry that using a PCIe 3.0 card in (one of) the bottom x16 slot(s) would negatively impact the PCIe 4.0 card in the first slot (assuming I'm using a board with a chipset that supports PCIe 4.0).

 

I cannot use my B450 board because all x1 slots are below the x16 slots and even getting a dual slot 6600 XT would leave me with a single x1 slot at most. Whoever designed the MSI B450 Gaming Pro Carbon AC deserves a special place in designer hell. Which is why I'm most likely getting a new board so that I can fit everything, so I might as well get one with PCIe 4.0 support.

 

52 minutes ago, mariushm said:

You could even use a pci-e riser cable to use a pci-e x1 slot that's often blocked by video cards otherwise and unused.

Wouldn't that need enough space in the case after the motherboard? Because the Suppressor F51 doesn't have that.

 

On aside note I do hope that all my studio USB audio still works without issues if I go that dual GPU route, because that has been causing trouble in the past when connected to the wrong port/controller or having an NVIDIA card in the system.

[urbanhusky]

After reading through a couple of motherboard manuals it seems that, at least in that small sample size, any additional mechanical x16 slots are connected to the chipset and therefore independent of the CPU lanes. The boards I had a look at had the second x16 slot running at x4, which isn't a problem in this case.

 

Anyway, YMMV, study the manual of your motherboard.

Thanks!

[mariushm]

These B550 chipset based motherboards split the x16 into  2 pci-e x8 slots :

 

ASUS ROG STRIX B550-E GAMING claims to do it but double check the manual

Asrock B550 Taichi  has 3 pci-e x16  and splits the first two slots into  x8  .. 3rd slot is pci-e x4 from chipset

ASUS ROG STRIX B550-XE GAMING WIFI does the same

ASUS ProArt B550-CREATOR does it

 

x570s  chipset boards - these 4 have  2 x8 + x4 from chipset   but note no pci-e x1 slots.

 

GIGABYTE X570S AORUS MASTER

GIGABYTE X570S AORUS PRO AX

GIGABYTE X570S AERO G

GIGABYTE X570S AORUS PRO AX 1.1

 

cheaper x570 boards with 3 pci-e x16 slots but you have to check manual to see how they're split (kinda sorting them by price, as i copy paste from a local It store that has nice filters and I can select only boards with the amount of pci-e slots i care about)

 

GIGABYTE X570 AORUS PRO

GIGABYTE X570 AORUS ULTRA

ASUS PRIME X570-PRO

ASUS ROG STRIX X570-F GAMING

GIGABYTE X570 AORUS MASTER

ASUS X570 ROG CROSSHAIR VIII HERO

ASUS X570 ROG CROSSHAIR VIII HERO (WI-FI)

ASUS X570 ROG CROSSHAIR VIII DARK HERO

 

Most of these have 1 or 2 pci-e x1 slots , asus prime has 3 .. but check manual to see which of them work when you populate x16 slots and m.2 connectors.

 

x570 boards with  x16 / 2 x8  + x4

 

MSI MEG X570 UNIFY

ASRock X570 Taichi

ASUS ROG STRIX X570-E GAMING

MSI MEG X570 ACE

ASRock X570 Phantom Gaming X

ASUS ROG STRIX X570-E GAMING WIFI II

MSI PRESTIGE X570 CREATION

 

All the above have 2 pci-e x1 slots as well except Prestige which has 3 - but keep in mind some x1 slots may be disabled when you use m.2 connectors.

 

MSI MEG x570 GODLIKE has 4 pci-e x16 , but it's x16 / 2x8 + x4 + x1 so not worth the extra dough.

 

 

 

In theory you can convert a M.2 (pci-e x4) to a pci-e x4 slot through a riser cable, and connect usb controller to it. There's such adapters available for purchase.

 

[urbanhusky]

2 hours ago, mariushm said:

These B550 chipset based motherboards split the x16 into  2 pci-e x8 slots :

Why would I need that?
I'm perfectly content with having the second GPU run with x1 speed via the chipset (because I will populate all PCIe slots and most manuals state that the second x16 slot will then run at x1 instead of x4).

[mariushm]

You said you also want two usb 3 controllers ... 

The number of actual pci-e lanes provided by chipset is limited, you only have 8 on B550 - so on some motherboards the pci-e x1 slots get disabled when you use the 2nd m.2 connector with a nvme drive, on others the bottom x16 (x4) electrically gets downgraded to x2 or x1  ... basically you may not have 2 other pci-e slots for those extra usb controllers.

Though there's one usb controller in the cpu (along with 2 sata ports, if they're not disabled)  and there's a usb controller in the chipset , so you do have at least 2 separate usb controllers

[urbanhusky]

I only have one M.2 NVMe SSD and I do not need a second M.2 drive (especially since, like you said, that often impacts PCIe slot availability).

I have two PCIe x1 controller cards: one for USB 2.0 and one for USB 3.0, because USB is hell and I have at least 14 devices, most of them are USB 2.0.

I have three SATA devices. Most boards have at least 4 usable SATA ports when all PCIe slots are in use.

 

So from what I can tell most B550 motherboards I looked at should work just fine. I have no intentions of overclocking or upgrading the GPU any time soon. An X570 board could yield a second usable M.2 slot perhaps, but right now I'm happy with what I have (except the GPU, which is the reason for this whole ordeal).

 

 

 

 

[Quantum Noisemaker]

5 hours ago, urbanhusky said:

After reading through a couple of motherboard manuals it seems that, at least in that small sample size, any additional mechanical x16 slots are connected to the chipset and therefore independent of the CPU lanes. The boards I had a look at had the second x16 slot running at x4, which isn't a problem in this case.

 

Anyway, YMMV, study the manual of your motherboard.

Thanks!

Yep, in this case I'd just read the manual religiously to make sure the board could support all the IO + graphics lanes you need. Watch out for the common gotchas, like M.2 deactivating x4 or multiple x1 lanes, etc. x570b would give you more PCIe lanes to play with, but it's definitely possible to find a B550 board that could do this. Also check rear iO and make sure you aren't losing too many USB ports there to allow for more PCIe connectors on the board. 

 

650 W on the power supply ay be a little tight. Depends on how much wattage your peripherals on USB draw and how effective the 480 underclock is. Definitely doable tho, just keep an eye out for system instability of course. If you see any of that behavior, you will have a likely culprit.