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Boosting the Mega ST motherboard

Posted: Sat Sep 23, 2017 9:22 pm
by guus.assmann

Here's the file, the schematic will explain most.
The Video part works fine, I used a 74AS00.
But when I try the rest, I get a crash screen. A row of bombs with a vertical bar on each of them, all the way to the bottom, from the bomb down.
Or the screen stay white for about 10 seconds an the the computer reboots.
Used a TOS1.04.
And 60nS Ram, F244 and F373 to replace the LS.
And of course a 16 Mhz CPU.
The MFP gets either 4 or 2Mhz, reaction is identical.

Any ideas on how to solve this?
The crash-screen looks odly fammiliar, I've seen it before but my memory fails me.

The mailserver(s) keep stating that your mailaddress is non-existant.
Using this forum works fine for me though.

Re: Boosting the Mega ST motherboard

Posted: Sun Sep 24, 2017 12:27 am
by exxos
OK so its clear now you mean 16mhz all the time not boosted 16mhz.

That mod is very difficult to get stable. I wrote about it on my website some time ago. The mega st has more noise problems than the stfm.

That mod is something I am working towards fixing but some time away yet unfortunately.

There are multiple problems to fix. For example the clocks all need buffering, but you simply can't buffer them as just a few ns skew causes the glue to malfunction. So all the clocks have to be replaced and regenerated.

Some people get lucky with this mod, but most end up with it unstable. In fact troed has been kitting out a test machine with this mod to help me along as I haven't much time to work on it on my own. As a side note, I am close to integrating it into my next v2 booster design.

Sorry I can't be much help. This mod and getting it stable is on my "to do list". Though there is a reason this mod isn't produced as a kit yet, and I think you found out why ;)

Also using F series logic is going to make your problems worse. You should never use F series on a ST, only LS series.

You also need fast tos chips, 100ns or faster. I use 55ns chips. Probable need to replace the bus pull ups resistors all to 2.2k. Of course recapped PSU as minimum. Resisistors in series with clock lines. Decoupling caps across each ic. I can advise thing to try to fix it, but I have not yet got as far as to debugging it myself yet,

Also don't forget the MMU gets double clocked. Not all mmus will be happy about it. I plan on replacing the MMU with the smt version as its a later production run, so stands a better chance of over clocking.

I also really suggest doing the mod on a stfm as they are more stable than the mega.

Boosting the Mega ST motherboard

Posted: Sun Sep 24, 2017 8:42 pm
by guus.assmann
Hello Exxos,

All pull-ups have had a 10K in paralelle to the 4K7. The result is about 3K3.
The Tos does make sense, hence my try to order your TOS-kit. I figured is would be faster.
There´s 2 boards already that act the same, so that´s offering some hope :-)

The F-types were put in, so as to also get a Pak68-2 and the 68020 boosters running.
With the LS in there (and the original 120nS Ram) both boards will not run at all.
With the F-types and 120nS Ram they do work stable, sort of anyway.
Neither board is tested for the FPU ok when I use Gembench 6. (Don`t know the version)

For all tests, I use your power supply.
Among other things, it will also stand being switched on and off very often.
A switch mode supply may not handle that too well.

The Mega-boards are used because:
1) I have many of those, 6 or more.
2) There´s a lot of room in the casing.
3) The shifter is mounted in a more convenient way.
4) Most of them are easy to expand to 4MB, although when using a 72 pin module, it`s the same.
5) I have various modules and expansions that will fit the Mega-boards and not the STFM, like TOS2.06, IDE interface, 12MB ram.
6) I want a Monster-Board to work along with the 16Mhz board if possible.
Yes, most of this is in a Falcon or a TT.
But you also found out I think. What´s the fun in that ?


Re: Boosting the Mega ST motherboard

Posted: Sun Sep 24, 2017 9:27 pm
by exxos
Did you change all the address & databus resistors ? some are normally 10K and 4k7.

I had the same mod running with my 4MB MMU kit, 60ns DRAM, I never changed the LS types. I have tried in the past, but F types always cause more trouble. I did a lot of testing in my MEGA ST with those in fact, don't know if I published the info.

Re: Boosting the Mega ST motherboard

Posted: Mon Sep 25, 2017 8:36 pm
by guus.assmann
Hello Exxos,

Then I'll have to check for the resistor values.
I've places 6 pieces 10K in paralelle to 4 resistor packs next to the Rom's and 2 next to the connector / processor.
On some boards I also replaced the Sil's with 3K3, all of them. But that's more work and risky.
There's only 2 LS244 in the boards I've done so far.
However I also own at least 2 boards with LS244's next to the MMU, to buffer the address lines.
Have not touched either of those yet.
But to make it simple, I have put sockets in all the boards that I've replaced the LS-es in.
Also, the CPU is socketed. And I'll have the Floppy controller in a socket as well.

I'll keep you posted of any results.


Re: Boosting the Mega ST motherboard

Posted: Mon Sep 25, 2017 10:34 pm
by exxos
It's possible 3k3 may not work. I know I tried various values, and 2.2K was the best value to use on my STFM. I use the same on all my kits, including the STE.

Likely you are fighting noise with the 32mhz input to the MMU ? also likely you will need to add something like 33R-68R on the clock outputs of the MMU, and any clock lines input or output to solve the ringing... check all the lines with a scope..

Boosting the Mega ST motherboard

Posted: Tue Sep 26, 2017 9:47 am
by guus.assmann
Hello Exxos,

Thanks, I'll have a good look. My scope is fast enough, it's a Tektronic, good for 250Mhz.

It just occured to me, there's a degree of success anyway.
The circuit to have the correct display works just fine.
Even when the board is running 16Mhz, the display is still correct size and resolution.
So trying 10Mhz or 12Mhz should be simple. It doesn't need a video sequencer.

One more question: How does the fast Rom access work?


Re: Boosting the Mega ST motherboard

Posted: Tue Sep 26, 2017 12:08 pm
by exxos
guus.assmann wrote:
Tue Sep 26, 2017 9:47 am
One more question: How does the fast Rom access work?

It works because the CPU is running at 16 MHz, so the CPU will read ROM at 16MHZ also, this works because GLUE issues DTACK about 20ns after address strobe goes low. This is exactly the same as I do with my V1.5 booster. Of course you need faster ROM ICs because of the high speed. They need to be faster than you would expect, because do not forget you have delays with the address decoding and chip enable lines. So you should try and use 100ns or faster. I use 45ns & 55ns chips.

The only real difference between V1.5 booster and the mod you are attempting, is that the RAM also accesses at 16 MHz, so of course you have to overclocked the MMU and of course have faster RAM.

Because of overclocking not all MMU's may function correctly at the higher speeds. It is something I have not tried yet, but technically I think the IMP set will be better to overclocked, but it will also cause more noise which you will have to solve anyway. You may find one MMU works more stable than another one, but of course you also have a lot of noise to contend with as you are running a lot more on the system at high speeds.

While it would make sense to use the F-series buffers, as they are faster, they also respond to noise faster and you ultimately get more glitches. I have tried other series of chips also such as HCT, ACT, HC, etc they are all generally faster than LS, and I had nothing but trouble with them, even with a stock machine. I also had similar issues with my STFM.

The only possible chips which may well which are faster, are with schmitt inputs, but very few ICs have this input. I have not checked, you should really try news this type as much as possible, then it will help snub the noise on the signals, and generally it could be a faster IC which may or may not help.

Re: Boosting the Mega ST motherboard

Posted: Mon Oct 02, 2017 10:50 am
by guus.assmann

Some success last weekend.
I managed to get the desktop working in low and medium resolution.
But I couldn't read the Ultra-Satan and also, High res would not start.
I had to select a MMU to get it going. And then I would also get a picture and desktop in High-res, but with "Jailbars"
Changing the GLUE made no difference.
And I could leave the MFP at the higher clock of give it a 2Mhz from a FF.
Most likely, the Ultra-Satan doesn't work because of the too high frequency, I've not checked how the signals are routed.
My conclusion is that I cannot get it stable. Maybe I'll try some more with different IC's, I still have the F373 and F244.
It could be that LS is better?
I've used the Tos-card from the store. And there's 60nS Rams on the board. (SIM with short wires)


Re: Boosting the Mega ST motherboard

Posted: Mon Oct 02, 2017 12:57 pm
by exxos
LS being slower acts as a noise filter. F series is fast enough to react to the noise and will cause glitches and oscillations.