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Author Topic: 6Mhz and more... (Read 23428 times)

Offline Zozosoft

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6Mhz and more...
« on: 2014.February.25. 23:28:18 »
The hw design of the Enterprise is originally ready for 6MHz model. The most important evidence about it is the option bit in the Dave (at port BFh), set the Dave sound frequency divider for the 4 or 6 Mhz system.
Very strange - why was 6Mhz machine not released?
Originally only 3 modifications needed:
-Z80B instead of Z80A
-12MHz crystal instead of 8MHz
-modify one bit in the EXOS code, for setting right frq for Dave

Unfortunately the Enterprise company made later a big mistake with the EXDOS: the WD disk controller does not have an own clock generator, it uses the system clock from the machine instead. So with 6MHz model the EXDOS cannot work...
I think this is because of cost reduction... but make some additional work when want to make a Turbo Enterprise.

See the schematic.
The 8MHz System Clock come from U18 Pin 12, need to cut this output line. Unfortunately it is hard to access, the best way is removing U18 and restoring it after cutting.
10316-0
10318-1
10320-2

Next: cut the System Clock from the Expansion Connector then connect the A23 pin to original 8MHz clock at the U18 pin12.
10322-3
10324-4
10326-5
10328-6

Then a new clock source is needed, 5V TTL compatible oscillator is the easiest solution, can be piggyback on U8, GND and VCC pins connected to the IC GND and VCC pins.
The clock output goes to the U18 pin 1 which are the input for the new System Clock.
10330-7
10332-8
10334-9

Replacing CPU to Z80B is optional, most of Z80A CPUs work on 6MHz. Try it and if it doesn't work then replace it. (I have never replaced it for 6MHz!)

The 7.12MHz turbo is a cheaper trick because no new clock generator is needed, using the 14M Video Clock!
All same as above, just instead of the oscillator output, connect the U37 Pin 10 (Video Clock) to U18 Pin 1 as System Clock. Many Z80A work at 7.12MHz but it is more possible that it needs to be replaced.

On some motherboards some problem can be found with the video memory on turbo speed, then short circuit R12. On higher speed (10MHz :-) ) also needs removing C7.
In 128K machines there also might be a problem with the internal 64K expansion, then short circuit R151. It will be good up to 7.12MHz CPU, but for 10MHz new super fast SRAM expansion should be bought from Saint :ds_icon_cheesygrin:

It is a good idea to add Turbo Switch because not all the games are better at faster :-)
Very ugly solution :oops: just add directly a switch to the clock lines: one input from U18 pin 12 (8MHz) another from the new clock generator or the Video Clock, and output go U18 pin 1. It is worked to me for many years in many machines :oops:
The next lesson will be: how to build electronic switch with 74LS157 IC :-)

Final words about the replacement of Z80: buy least 10MHz model (Z84C0010), but don't order it from China! Previously I wrote my experiences, the Chinese sellers sold relabeled old and slow Z80 CPUs :-(
« Last Edit: 2014.February.26. 13:04:49 by szipucsu »

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #1 on: 2014.February.26. 11:14:43 »
There is a schematic for 74xx157 as switch. You can see it on the last photos.

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #2 on: 2014.February.26. 11:36:40 »
Some other modification: with EXOS 2.3+ possible detect with phantom bad segments, especially on a turbo speed. This because the data bus don't have pull-up resistor inside the machine then it is floating. Some external devices for example the MICROTEAM EXDOS card have a onboard pull-up resistors then when using the machine with this card the problem solved.
I don't fix it the phantom segment detection in my RAM test module, because it is a good sign of floating databus problem! It is can make problem with many games which are using IM2 interrupts but not using complete IM2 table. If at the IRQ not FFh coming from the data bus (because the floating) the game will be frozen...

The best way solve the problem add pull-up resistors to the mainboard, at U7 (74LS245) data bits at Pin 2 to 9, +5V at Pin 20:
10338-0

Offline jltursan

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Re: 6Mhz and more...
« Reply #3 on: 2014.February.27. 20:33:03 »
Thanks Zozo, right in time!. Although I'm very busy lately, I'm already planning my next spare time with my upgraded EP64 and overclocking was the first in my list :-)

I'll read it more carefully. Need to double check the 74xx157 installation and 10Mhz details, not sure at all about how to get this speed.

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #4 on: 2014.February.27. 21:08:17 »
For the 10MHz you need:
- 10MHz Z80, part number Z84C0010PEG/PEC
- 20MHz 5V TTL crystal oscillator, for example this.
- fast memory expansion from Saint
- fast EPROM/Flash ROM chips for EXOS/EXDOS/other ROMs, below than 100ns recommended
- 74F157 for a switch

Cuts and wiring see previously. R12 needed to short circuited and C7 removed.
« Last Edit: 2014.November.04. 01:29:27 by Zozosoft »

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #5 on: 2014.February.27. 21:28:22 »
157 installation: you can see on the photos, and schematic.
it is piggyback on the onboard multiplexer (157 or 158), pin 8, 15 and 16 connected to the onboard IC, these are the GND and VCC pins.
Other pins are "floating" in the air.
Pullup resistor added between pin 1 and 15. Pin 5 connected to U18 pin 12 (8MHz), Pin 6 to oscillator output (Turbo Clock), Pin 7 to U18 pin 1 (System Clock).
I added a connector for the switch, because the switch glued to the case under the keyboard. The connector will be good if needed to remove motherboard from the case.
Connector pin 1 at the 157 pin 1, connector pin 2 to GND.

If you use two circuits switch then you can made a Turbo LED: replace the power LED with bi-color LED (3 pins type), one color conected to the power led pins, second color controlled with the switch.

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #6 on: 2014.February.27. 21:38:53 »
First I thought Dave needed a heat sink. But now I looked at it after running a few hours at 10MHz, it is only about a hand temperature! I think about 40 celsius.
And the Z80s are cold! :-) at only room temperature. CMOS rulez :-D
« Last Edit: 2014.February.28. 09:42:14 by szipucsu »

Offline jltursan

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Re: 6Mhz and more...
« Reply #7 on: 2014.February.28. 11:20:31 »
Great, thanks again!. what about the onboard RAM, I have saint's expansion but also old 64kb in my ep64.

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #8 on: 2014.February.28. 11:27:23 »
Onboard RAM accessing controlled by Nick and using fixed timing. Only some adjust needed this the short circuited R12 and removed C7. If it is with 150ns ICs then will work with 10MHz.

Offline jltursan

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Re: 6Mhz and more...
« Reply #9 on: 2014.February.28. 19:32:30 »
Great, need to check them tho, I'm not sure about their speed.
I suppose the 10Mhz mod is compatible with slower Mhz CPUs just replacing the 20Mhz xtal, isn't it?, I'm asking that because I'm not sure about the 10Mhz Z80s I already have, they come from China and although they work fine replacing standard ones, if they're fakes they indeed can fail running at higher speeds :-(

About the ROMs, I've just ordered 10 W27C512-45 pieces. They're cheap and I hope being 45ns, they would be enough fast! ;-)

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Re: 6Mhz and more...
« Reply #10 on: 2014.February.28. 19:46:14 »
Yes, you can use any slower crystal for slower CPU speed. For a 7.12Mhz CPU not needed additional oscillator just use the Video Clock as System Clock.

Offline jltursan

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Re: 6Mhz and more...
« Reply #11 on: 2014.June.19. 11:09:55 »
Well, it's done!

10662-0

Notice the green led, I followed Zozo's suggestion and replaced the original led by a bicolor one, it's very handy to know when you're in turbo mode!

I'm still suffering the RAM segments bad detection; so I'll need to add the pullup resistor array...

Thanks Zozo!
« Last Edit: 2014.June.19. 13:32:27 by szipucsu, Reason: Linked image uploaded here »

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #12 on: 2014.June.19. 14:05:04 »
Quote from: jltursan
Well, it's done!
:smt038

CPU still the original or replaced? (Many Z80A can run on 7.12Mhz :-) )

This is the 032383 S/N machine? I will update notes at inventory :-) When adding the pullup resistor can you view the motherboard ISSUE?
« Last Edit: 2014.June.19. 14:14:09 by Zozosoft »

Offline jltursan

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Re: 6Mhz and more...
« Reply #13 on: 2014.June.19. 15:24:50 »
Yes, the machine is the 032383 S/N one.
The CPU is still the original, it doesn't get very hot :). I'm waiting for some thermal pad, then I'll add a heatsink just to be safe.
EXOS eprom was changed after severe unstabilities. Using the above W27C512 solved the problem. And finally, I also needed to replace (again!) the EXDOS controller EPROM by another W27C512, burning twice the EXOS 1.3 file. Without this new EPROM, EXDOS card behaved very strangely, the floppy was, from time to time, rattling the heads :shock:

About my EP64, where can I check the issue? (btw, can't check Nick & Dave revisions as there's a heatsink glued to them)

And given your photo, the pullup resistors are 4K7Ohm?

Some more pics:



I've piggybacked 157 over U9; but you can also see a DIP14 socket over U8, I've also installed this to be able to mount a new oscillator and try with different speeds, specially if I finally replace the CPU with a CMOS one.



Also, I managed to cut U18 trace without desoldering the chip, just a small sharp cutter is needed :)

All in all, it was an easy mod (no memory or CPU worries :smt023)

Next: Saint's super interface and who knows...maybe the floppy HD mod?. I'm also interested in coding some assembler

Offline Zozosoft

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Re: 6Mhz and more...
« Reply #14 on: 2014.June.19. 15:40:22 »
Quote from: jltursan
About my EP64, where can I check the issue?
It is written on the motherboard bottom side at the modulator :-)
From top view: check the monitor connector, if all pins are present then it is ISSUE 6.
If some pins are missing, then check the company name at the left side of the mb. If it is ENTERPRISE then it is ISSUE 5, if FLAN then it is ISSUE 4.
« Last Edit: 2014.June.19. 18:40:49 by szipucsu »