50MHz Turbo Color Slab Clock Doubler PCB

[Nitro]

This is a community work log to create a printed circuit board for the clock doubler used in the 50MHz Turbo Color Slab mod. Feel free to join in on the discussion and help to design a PCB to make overclocking easier. The new board will be based on the clock doubler circuit and SIP prototype PCB created by forum user @gborgns.

Here's a picture of the current board and schematic:

Link: Homemade clock doubler

Doubler based on Quaddoubler with the exact circuitry listed in the Motorola spec. sheet on the MC88916. I used a SIP prototype PCB with a copper tape ground plane added on the bottom. Works as advertised.  Note that the IC is upside down in the picture.

Link: Schematic for homemade clock doubler

Schematic for the homemade clock doubler.  All capacitors are ceramic and resistors thick film.

It's difficult to see from the photo but some of the traces on the prototype board have been cut, so it's a daunting task to replicate the current board for a hobbyist like me. If anyone has the skillset to lay the board out then please do, if not I'll try my hand at it if time permits. I've been studying KiCad at YouTube University and I'm still not sure I'll be able to do it.

I don't know the best way to interface with the motherboard, maybe use header pins, through-hole solder pads or small terminals? Mounting holes in the four corners for plastic standoffs? Best way to add a ground plane? Let me know what you think.

[gtnicol]

What is the result of using that clock doubler? Is that something you could use on a turbo cube board, or could you do something similar on a regular 25Mhz board?

[Nitro]

What is the result of using that clock doubler? Is that something you could use on a turbo cube board, or could you do something similar on a regular 25Mhz board?

From my understanding it takes a 25MHz reference signal and doubles it, and that signal is used to run the CPU and bus at 50MHz. The 33MHz CPU is commonly swapped out for a 40MHz unit for a better chance of stability. I don't know if this would work on the turbo cube or the 25MHz boards. I'm guessing that it would work on the turbo cube if you could determine the tie-in points on the motherboard.

[zombie]

I believe there was an upgrade, was it called the pyro? that doubled 25mhz machines to 50mhz, and if I recall worked well. It would run faster than turbo machines overall on most things.

There also was some next official upgrade, I think the nitro, that brought machines up to 40mhz, and I think it outpaced the pyro in most but not all ways.

What I wonder is has there ever been an attempt to double the turbo machines, ie a 66mhz doubling of the 33mhz machines?

[cuby]

What I wonder is has there ever been an attempt to double the turbo machines, ie a 66mhz doubling of the 33mhz machines?

This would require significant overclocking of the 68040 - I think the fastest official rating was 40 MHz, so this is unlikely to work (unless somebody wants to try a liquid cooler their 68040 ).

[nuss]

I believe there was an upgrade, was it called the pyro? that doubled 25mhz machines to 50mhz, and if I recall worked well. It would run faster than turbo machines overall on most things.

There also was some next official upgrade, I think the nitro, that brought machines up to 40mhz, and I think it outpaced the pyro in most but not all ways.

What I wonder is has there ever been an attempt to double the turbo machines, ie a 66mhz doubling of the 33mhz machines?

There may be more performance-improving hardware differences between Turbo and Non-Turbo machines. But to my experience the Pyro performance gets already limited by the Non-Turbo RAM speed (100ns), therefore the Turbo machines out-perform it in many day-to-day cases, when using faster RAM (60ns). Compiling times (depending also on HDD) I do not remember, but for pure number-crunching the 50MHz Pyro (monochrome Cube) was faster compared to the 33MHz Turbo slabs. At least it was so in my tests in the 90s

Also I thought the very few Nitro-upgrade-kits for Turbo machines were only prototypes and not available for official purchase?

Of course I am not a NeXT insider, but I do not remember (from any discussions and publications in the 90s) that a 66MHz upgrade for Turbos was planned (for sure at our institute we would have been interested in such performance gain). Anyhow there was discussions (or rumors?) about using 68060 to increase performance.

[zombie]

I agree with your well informed thoughts @nuss and that is also my memory of things. Yes, the nitro kits were never made available but I believe one or two of the forum members actually have one, and they are reported to work well!

I believe there is even one or 2 MPEG compression chips available to fit the NeXTdimension boards. I believe one user tried them and it crashed immediately, and that the MPEG chip, sadly, was as far as we know, never made to work.

I have heard of the (I think) 8800Risc brick machine too. I believe there was mention of prototypes. But I've never heard/seen one that was working. No doubt somewhere inside NeXT it was, but I havent seen any escape into the wild in working order.

[zombie]

Oh, and as for the 68060 chip, from what I've heard there is some incompatibility. I guess some commands or something that the NeXT relies on in the 68040 that does not get invoked correctly by the 68060.

The latest I think I've heard of on that front are FPGA based chips that can perfectly emulate the 68040 and run 10s of times faster. That is in theory and I believe some have been made to work on the Amiga platform, but have never seen this effectively brought over to the NeXT side of things.

[gtnicol]

I believe there was an upgrade, was it called the pyro? that doubled 25mhz machines to 50mhz, and if I recall worked well. It would run faster than turbo machines overall on most things.

There also was some next official upgrade, I think the nitro, that brought machines up to 40mhz, and I think it outpaced the pyro in most but not all ways.

What I wonder is has there ever been an attempt to double the turbo machines, ie a 66mhz doubling of the 33mhz machines?

I've had a few Pyro's and overall they made things a bit faster, but I generally feel the Turbo Mono feels better. I never booted my Nitro

[zombie]

Why not! BOOT IT BOOT IT BOOT IT!

[Nitro]

gborgns mentioned benchmarks of his 50MHz mod here.

https://www.nextcomputers.org/forums/index.php?topic=56.msg7771#msg7771

And Pyro/Nitro benchmarks are here.

https://www.nextcomputers.org/NeXTfiles/Docs/Rare_NeXT_Hardware/pyro_results.pdf

If you decide to fire that Nitro up don't let the smoke out!

[cuby]

Oh, and as for the 68060 chip, from what I've heard there is some incompatibility. I guess some commands or something that the NeXT relies on in the 68040 that does not get invoked correctly by the 68060.

There are more complications... and, of course, there was a thread about it some years ago .