Bottom line: With third generation Threadripper processors set to arrive later this month, AMD has vowed to provide long-term support for the new sTRX4 socket. This is a good thing because the high-bandwidth sockets are not cross-compatible with previous generation sTR4 motherboards that were used for the first- and second-generation Threadrippers.
Third-generation Ryzen Threadripper CPUs were announced last week. On Friday, AMD posted a video spotlight on the HEDT Treadripper 3960X and 3970X (watch below), both based on AMD’s 7nm Zen 2 architecture.
Threadripper 3960X will have 24 cores/48 threads, while Threadripper 3970X packs in 32 cores with 64 threads. The new CPUs rely on a new sTRX4 platform, which means they will only work on new TRX40 motherboards. Unlike Ryzen AM4, Threadripper is not backwards compatible with the sTR4 socket used in the first and second-generation Threadripper.
Even though the sTRX4 socket has the same number of pins (4096) as sTR4, they are electronically incompatible. According to AMD, the pin mapping to voltage or data is entirely different. Therefore, you won’t be able to plug a third-gen Threadripper into your existing X399 motherboard, but this is not without what AMD considers good reason.
"AMD's sTRX4 socket isn't being replaced anytime soon."
“There are two essential reasons for this,” AMD said in the r/AMD subreddit. “[First,] we wanted to drive maximum performance for the 3rd Gen AMD Ryzen Threadripper processors, and sTRX4 helps us do exactly that. [Second,] the socket change also sets us up nicely for future development and scalability of the Threadripper platform, both on a near- and long-term basis (emphasis AMD).”
The company explains that third-generation Threadripper processors will have a total of 88 PCIe Gen 4 lanes with 72 usable (CPU + motherboard).
“The net of total versus usable is because we’re also increasing the CPU<->chipset link from 4x Gen4 to 8x Gen4—quadruple the bandwidth vs. 2nd Gen TR. Extra data pins between the chipset and CPU make this possible, so you’ll be able to hang more I/O off the motherboard at full performance.”
AMD apparently intends to support the sTRX4 socket for the long term as it will encourage enthusiasts to invest in TRX40 mobos, knowing that they will not have to worry about switching in the near future. Contrarily, Intel changes sockets every couple of years.
Third-gen Threadripper is set to launch on November 25 with the 3960X going for about $1,400 and the 3970X at about $2,000.