Thought about doing this with mine before now, but never had the balls to do it and built a 386 instead.

In theory though you're right, except if something else derives it's clock from the 20MHz Oscillator. IBM and Sega are both companies which had a rather convoluted way of constructing things.

If you really want to try it, i'd suggest you desolder the oscillator and install a socket for those square oscis. Then you just need a new oscillator, which i reccomend you to be conservative at first. Try a 24MHz one and the CPU should not object working at 12MHz.

But like HighTreason said of anything else uses the standard oscillator there could be instabilities. 12MHz might not be a big push for the CPU but could be if the system bus uses it.

Looking at a complete picture of the board, there is a 14.31318MHz osci which was standard in the IBM PC architecture, so i think it is quite safe to say the system bus will use this osci.

Again i'd recommend you if you really want to do this be conservative. Get a 24MHz osci and try that. The CPU is unfortunately not socketed so there's no way to say if the CPU could do 16MHz without any instabilities.

If you can do SMD precision soldering, then you can try to solder a new CPU or better yet, a socket. A 20MHz 286 will be fast enough for a lot of DOS games.

Good luck.

^ This, I was about to write almost the same thing..

I'd have to take the time to look closely, but my concern would be the MC68K using the same Oscillator, it shouldn't though, there should be a 53MHz one for the Mega Drive side - if there is it should be wired similarly to a regular Mega Drive leaving you free to tamper with the PC side without affecting it.

Why not get another OEM 286 or 386SX? Might be a lot easier.

Or buy/build generic? Even easier and you won't have a stack of soldered or proprietary parts when you inevitably want to upgrade something in that?

Thanks for the replies.

I wonder to what extent the PC side interacts with the Sega MegaDrive side other than for the Puzzle Construction program. I took a look at the datasheets of the nearby PC Ic's such as the Western Digital WD76C10LP system controller, and I'm pretty sure this chipset uses the same OSC. , but can also handle up to 25mhz speeds.

HighTreason wrote:

Thought about doing this with mine before now, and built a 386 instead.

I though of that as well, but I'm just going to keep everything 16 Bit.

carlostex wrote:

Looking at a complete picture of the board, there is a 14.31318MHz osci which was standard in the IBM PC architecture, so i think it is quite safe to say the system bus will use this osci.
If you can do SMD precision soldering, then you can try to solder a new CPU or better yet, a socket. A 20MHz 286 will be fast enough for a lot of DOS games.

Yes, there's a 14.318Mhz OSC, and there is also a 48Mhz xtal nearby. The MD side seems to be covered with the common NTSC 53.6931Mhz Master CLK OSC.
My idea is to go with a CPU socket, then use the N80L286-16/S and replace the 20.000MHz OSC for a 32.000Mhz one.

HighTreason wrote:

I'd have to take the time to look closely, but my concern would be the MC68K using the same Oscillator, it shouldn't though, there should be a 53MHz one for the Mega Drive side - if there is it should be wired similarly to a regular Mega Drive leaving you free to tamper with the PC side without affecting it.

As of right now I'm only going by pics, once I get into the PC and take it apart I'm going to trace everything to make sure. Speaking of the MC68K, the one in the Teradrive also seems to be rated at 10Mhz, which its kind of unusual for a MD, but I really hope it is wired just as in a Sega MD.

Keep in mind that for successful overclocking, you might need to change the RAM chips too. I have 41256-10 (10 nsec DRAM) onboard chips for my 10 MHz 286. For a 12 MHz version one would need a 42156-8 (80 nsec DRAM) or a motherboard that supports adding memory wait states (not I/O wait states). As far as I know, for faster 286s, the only option is to go with the wait states (although I've seen in older magazines that 41256-06 (60 nsec) DRAMS exist, however they are very rare to be found nowadays if ever).

I don't think I would risk that. IMO chances are too high it either doesn't work or the machine gets damaged which would be a pity, especially considering rareness/collector's value.
Also I mentioned on your other thread about the machine that the on-board VGA might hold it back. It seems to be VGA only (not SVGA) with 256kB of RAM so I don't suppose it is particularly fast. Maybe you could run some benchmarks to see if performance is in line what could be expected by a 10MHz 286? I don't know if you can use an ISA VGA card, at least I would expect you lose the Megadrive output on VGA.
Overclocked I would expect the VGA to limit the machine even more. If you have a look at the mainboard, can you tell what VGA chipset it uses?

Anyway, I would just play those games on the Teradrive that run fine and play more demanding games on other machines.

idspispopd wrote:

Also I mentioned on your other thread about the machine that the on-board VGA might hold it back. It seems to be VGA only (not SVGA) with 256kB of RAM so I don't suppose it is particularly fast. Maybe you could run some benchmarks to see if performance is in line what could be expected by a 10MHz 286? I don't know if you can use an ISA VGA card, at least I would expect you lose the Megadrive output on VGA.
Overclocked I would expect the VGA to limit the machine even more. If you have a look at the mainboard, can you tell what VGA chipset it uses?

The Teradrive uses the Western Digital WD90C10-LR VGA chipset as seen in the link below (it is coupled with the WD90C61 chip that produces the clock signal).

http://nemesis.hacking-cult.org/MegaDrive/Ter … es/IMG_0842.JPG

I can't seem to find the actual datasheet for the chip but is very similar to the WD90C00-JK that can be found in other 16-bit ISA VGA cards, e.g.:

http://www.ebay.com/itm/Western-Digital-WD90C … =item35ea2daf25

http://www.ebay.com/itm/Tandy-1700455-C-87091 … =item4acc39c380

This is the WD90C61 datasheet, according to it the Master clock frequency for the WD90C10 should be 37.585Mhz

http://www.datasheetarchive.com/dl/Scans-068/ … 2IH00221848.pdf

So if I were to replace the CPU altogether will this affect the video too?

And here's a quick SYSCHK:

Usually faster CPU increases Video speed too.

Megadisk wrote:

The Teradrive uses the Western Digital WD90C10-LR VGA chipset as seen in the link below (it is coupled with the WD90C61 chip tha […]
Show full quote

The Teradrive uses the Western Digital WD90C10-LR VGA chipset as seen in the link below (it is coupled with the WD90C61 chip that produces the clock signal).

http://nemesis.hacking-cult.org/MegaDrive/Ter … es/IMG_0842.JPG

I can't seem to find the actual datasheet for the chip but is very similar to the WD90C00-JK that can be found in other 16-bit ISA VGA cards, e.g.:
[...]

So if I were to replace the CPU altogether will this affect the video too?

And here's a quick SYSCHK:

WD chips are generally not bad, the later ISA ones are really good. I can't find benchmarks for WD90C10 specifically, though. But if the WD90C10 is closer to WD90C00 as you suggest (the (C)WDC '89 on both chips agrees with that) then WD90C11 than it is not nearly as good as the later ones (judging from the http://www.vgamuseum.info/index.php/benchmarksvgamuseum.info benchmarks , WD90C11 is very close to WD90C31).

What I meant to say is, if the VGA can't take data as fast as the 286 and the ISA bus allow, it would become a bigger bottleneck with a faster CPU. The thing is, I don't know how fast a VGA card would have to be for a 286. Perhaps a 286 is already able to saturate the ISA bus, I just don't know.
The other issue is, will the ISA clock (which should also be the clock the internal VGA is accessed with since that is an ISA chip) be the same as the CPU clock, or will it be for example 14.31318MHz/2 = 7.15659MHz? It wasn't uncommon for 286 systems to use the same clock for CPU and ISA which won't be a good idea with 16MHz (might still work with 12MHz - I overclocked the ISA bus on a 386DX40 to 13.3MHz with a WD90C31, but that was later hardware than yours).

Regarding SYSCHK: The value of 5755 char/sec has to be through BIOS so it does not reflect the VGA chips speed very much. I don't know what "Throughput Speed" exactly means.

Wow just wow! 😲 .

This Japanese IBM 286 pc has exactly the same chipset as the Teradrive (VGA an all, with the few differences of the 12Mhz CPU, i287 co-pross, ram upgrade and more ISA slots)Damn you IBM 😠 .

Which further leads me to believe that Teradrive can definitely run much faster with th same chips and new CPU(at least at 12Mhz).

Click on the high res. links for better view.

http://translate.google.com/translate?hl=en&s … tml&prev=search

.

If the translation is correct the i287 is a mod, the option wasn't there originally. Anyway it isn't important to have an FPU in a 286, especially for games.
According to the diagram CPU clock is separate from bus clock (14.3MHz/2 as I guessed). Maybe you can compare the diagram to your mainboard?
The datasheet of WD76C10 is indeed rated for up to 25MHz according to the datasheet.
The NEC 424100-60 chips on that IBM probably mean they are they are 60ns chips.

Still, keep in mind that this one is 12MHZ, not 16. 12MHz would mean a mere 20% overclock which is probably measurable but not noticable, and the VGA might already be a bottleneck.

So you might have a chance, but still I wouldn't mod such a machine and just use a faster computer if you have to.

I think I'm going to go with the 286 12Mhz CPU substitute first, and see how it goes from there. Still, one part of me says just to leave it alone, while another part of me says :

The only game that utilizes a FPU on the 286 is SimCity. I have a FPU in my 286 12.5MHZ.

I wonder how good does a 12Mhz 286 handles Wolf3D for example.

PeterLI wrote:

Why not get another OEM 286 or 386SX? Might be a lot easier.

Yeah I have an Amstrad Mega PC that pretty much suits all of my needs, but I'd like for the Teradrive to get more action.

A quick update, after giving it some thought I decided to give this a go. Removed the stock CPU, the master clock oscillator and installed sockets. I have in hand some faster 286 cpu's (12/16Mhz) with matching OSCs and will test them soon. As per some of the comments above and just in case, what exactly does a 286 looks like with a bottleneck VGA?

I temporarely re-installed the original cpu to test my soldering skills and everything is working fine prior to the faster cpu replacements.

Megadisk wrote:

I wonder how good does a 12Mhz 286 handles Wolf3D for example.

i would think pretty well.

i cant discount the effect of nostalgia but our 10mhz IBM PS/2 286 ran it reasonably well with pc speaker sound and then later with opl3 music.

that machine is long dead but an extra 2mhz would have made it pretty smooth i would think.

Keep the bottlenecked VGA in mind. You might need a faster CPU to compensate.