ZigZagJoe's Vintage

NeXT OverDrive Accelerator

Presenting the first new accelerator design for NeXT black hardware. Operationally similar to the rare vintage Pyro accelerator, it doubles system clock speed to 50mhz.
Status: Under development with the related 040HyperDrive accelerator. Working prototypes. Stability test passed on Turbo hardware, non-turbo still requires additional stability work.

Each accelerator has a new CPU onboard capable of operation at this speed. Prototypes use a QFP MC68040 CPU. This design does not overclock the rest of the system and requires no modifications to most machines. For systems with a CPU socket it drops straight in. Non-socketed machines will, of course, need a CPU socket installed. These are somewhat hard to come by unfortunately.

Supports all 68040 NeXT computers at a design level. However, non-turbo cubes I expect to require a modified board design.

Turbo hardware requires a single jumper to be installed to reduce bus speed to 25mhz from 33mhz. However, due to the additional wait states added for 33mhz operation this doesn't actually result in performance loss.

Benchmarks

Here's some home-baked benchmarks of common tasks along with memory bandwidth as measured by membench.c
Definitely a nice bump for 25mhz systems and still a notable improvement on 33mhz systems. It can be seen that IO performance remains essentially constant vs 33mhz as would be expected with reduced wait states.

Overclocking?

My design does not overclock the system. The on Turbo systems only, you can use the BCLK overclocking method seen here to increase the system clock. This is running the system bus (BCLK) at 50mhz overclocking the entire system. This improves performance across the board, especially video, but stability would have to be assessed on a per-system basis as it's far out of spec. I have been told Turbo ASICs have a tendency to die due to unclear causes so overclocking may be risky in the long term.

Overclocking gives greater performance - possibly the best possible from black hardware - but is a very invasive modification. CPU must be replaced with a newer chip capable of supporting 50mhz + operation, as the early mask chips used by NeXT do not reliably operate beyond ~33 mhz. The logic board PLL must also be de-soldered and replaced with a faster rated version.

Development log #1
Development log #2