(Image credit: AMD)

Third-gen Ryzen's support for PCIe 4.0 opens up a new world of faster connectivity, but according to a recent statement from the company, you won't be able to enjoy the faster transfer speeds on older AMD motherboards, taking a bit of the shine of AMD's vaunted backward compatibility with Ryzen processors on the AM4 socket.

At CES 2019, word emerged from motherboard vendors that they could support PCIe 4.0 on older motherboards, like X470, X370, B350, and A320, and AMD confirmed that it would not restrict the faster standard on older motherboards. We've even seen the feature enabled on recent third-gen Ryzen-enabling BIOS updates, but now AMD has reversed the earlier decision:

Users today may find a PCIe 4.0 option available in their pre-X570 motherboards. However, users should expect this option to be disabled when final retail BIOSes are released to implement full performance and stability for new 3rd Gen Ryzen processors. As pre-X570 motherboards were not designed with PCIe 4.0 in mind, their designs may be incapable of running PCIe 4.0 signaling with the requisite stability and performance. To ensure a reliable and consistent experience in the field, PCIe 4.0 will not be an option ultimately available to pre-X570 motherboards. Users may continue with a beta BIOS if they desire, but performance and stability cannot be guaranteed.

The problem boils down to signal integrity. Motherboard vendors at Computex 2019 told us that motherboard traces, the electrical pathways that carry data signals, require wider spacing with PCIe 4.0 compared to PCIe 3.0. The wider spacing, coupled with the increased power requirements for the PCIe bus, necessitates placing the transmit and receive traces on different layers of the motherboard, whereas PCIe 3.0-capable motherboards can carry the signals on a single PCB layer. That means most new X570 motherboards, and their newer B- and A-series counterparts, will come with a minimum of six layers, while previous-gen motherboards would dip down to four layers. That has obvious cost implications that will manifest as higher pricing on new AM4 motherboards. (Edit: There is word that Gigabyte has a few four-layer boards, we're following up for clarification.)

PCIe 4.0 also requires other additives that support the higher transfer rates, like switches and mux layouts, and trace lengths longer than six inches won't be able to carry the signal. Those restrictions would have limited the PCIe 4.0 connection to the first PCIe slot on older motherboards, but several vendors also told us the first slot wouldn't operate at full speeds due to the closer spacing of the PCIe 3.0-designed slots, which would introduce signal integrity issues.

That means, even under the best circumstances, the first slot on older motherboards would only operate at a portion of the PCIe 4.0 throughput (akin to a "PCIe 3.8" or "PCIe 3.7" connection). The increased data errors on the line would also trigger PCIe's error correction mechanisms more frequently, which could introduce data integrity issues.

AMD also told us at Computex 2019 that older motherboards would require PCI-SIG certification to support PCIe 4.0, a costly expense for motherboards that are already in production, and a few vendors that had planned to retroactively enable the feature told us that they would only do so unofficially and not market the faster transfer rates as a feature on older motherboards.

Even on newer motherboards with correct trace spacing, PCIe slots that extend beyond six inches (think the last slot on the motherboard) also require redrivers, one for each lane, that add even more cost to the new 500-series motherboards. Be prepared to whip out the wallet for these new motherboards. In fact, several vendors told us that they anticipate such high pricing for 500-series motherboards that they will keep the 400-series motherboard in extended production as the value option for third-gen Ryzen chips. That doesn't bode well for A-series motherboard pricing with the new motherboards.

AMD will disable the feature in the next AGESA update for the third-gen Ryzen processors. You can opt to use the few motherboard BIOS's that already exist to enable support for "PCIe 3.8," but that's a risky proposition.

It's easy to be disappointed that AMD isn't delivering on its promise of full backward compatibility for the AM4 socket until 2020, but the company has already been forced to drop support for some chips due to BIOS restrictions. The company also isn't supporting third-gen Ryzen on all previous-gen motherboards. That's all part of the challenge of undertaking the monumental task of supporting all Ryzen processors on a single socket. Perhaps AMD representative Robert Hallock's statement to us sums it up best:

"No one in the history of x86 has created an upgradeable socket quite like AM4. In a time where our competitor is breaking socket compatibility yearly, basically, we have three consecutive generations that all drop into the same socket, and that socket started with four cores years ago, and is now twelve cores, twenty-four threads and PCIe 4.0."

Given the circumstances, AMD is making the right choice here. Complicated support matrices, unofficial (and thus un-warrantied) "support," lowered throughput, and possible data loss are all untenable side effects of giving users what they want -- PCIe 4.0 support on motherboards that weren't designed for the task.