Earlier this year, Caustic Professional began shipping the first dedicated ray tracing cards in more than a decade. As we discussed in our coverage last year, ray tracing is capable of producing some stunning visual effects. Multiple companies, including Nvidia, offer ray tracing plugins for professional workstations, but the Optix solution we covered then doesn’t leverage dedicated hardware to perform calculations. Caustic Professional’s R2100 and R2500 add-in boards, on the other hand, serve as dedicated hardware accelerators.
Meet the RTU
The R2500 uses a pair of RT2 ray trace units (RTUs). Each RTU is capable of calculating up to 50 million incoherent rays per second, and each chip has 8GB of dedicated DDR2 RAM (16GB total). There’s also a smaller card — the single-core R2100, with 4GB of on-board memory. The RT2 is a custom ASIC built on a 90nm process, but the card draws comparatively little power. Caustic, which is owned by Imagination Technologies, the same company that makes the PowerVR line of mobile GPUs, specs the R2500 at a max of 65W, while the R2100 can draw 30W.
Looking at the R2500, I’m struck by how much it resembles a 3dfx Voodoo 5 5500. Not so much the 6000 — I owned a working V5 6000 once upon a time, and that card was absolutely covered in copper traces. But the long, thin board, the rows and rows of memory — for a card that’s meant to usher in the future, it conjures a lot of memories.
Part of what keeps the R2100 and R2500 relatively svelte is that they offload object shading to the CPU rather than handling it on the card itself. Storing shader maps and texture data in system memory frees up resources on the card for the ray tracing calculations and model storage. Offloading such capabilities to the GPU could theoretically increase performance, but GPUs generally aren’t designed to share data in that fashion.
Here’s the ASIC itself; the RT2-ES1. It measures 11x10mm, and is built on a 90nm process. PLX bridge chip in the center of the card is a 27x27mm and the RT2-ES1 is smaller than that. You can see the RAM arrays around the chip clearly, but the only visible traces run from the RT2-ES1 ASIC to the PLX 8362 bridge chip.
The PLX 8632 bridge chip is a PCI-Express 2.0 controller with a total of 32-lanes of PCIe connectivity. There’s an x8 connection for each of the two RTUs and a x16 link back to the main system bus.
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