MTX new Replacement mainboard ??

Modern, Memotech inspired, hardware projects
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Crazyboss
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MTX new Replacement mainboard ??

Post by Crazyboss »

Hi

Wonder if it is possible (worthy?) to make a run of new Memotech Mainboards. I guess it should be possible using KICAD or something.

"JUST" put in the original design from the MTX user Manual ? I am not thinking about REMEMOTECH or MTX+ but a board which can be fit inside the original MTX Case, with the right connectors where they should be, also using the original hardware. Wonder what the cost for a project like that could be, I bet its possible to get the PCB's with already sockets onboard when you order?

Also if possible someone could rearrange the things on the board e.g move the power supply circuit to a better place. Or maybe optimize a bit around maybe there are other ic's around today which was not there in the 80's, which could simplify the mainboard.

So what I am talking about is a replacement board, with all the original functions, like can connect to FDX, REMEMORIZER, TAPE, JOYSTICKS and PRINTER.

I know a Colecovision guy was working on a new PCB for ColecoVision ;)
//CLAUS - Webmaster at www.mtxworld.dk
lezanderson
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Re: MTX new Replacement mainboard ??

Post by lezanderson »

There's several options to making an MTX replacement motherboard.

1) reverse engineer the original as Claus has said.. but this option would need to Start from scratch and would give no improvements over the original ? Plus it would probably be the most costly option!

2) Modify Dave's and Martin's MTX plus to function as an Enhanced original MTX motherboard this may be the easiest and cheapest option.

3) Design a New Motherboard based around an F18A.

Each option has it's Positive and Negative points, but option 2 is probably the most realistic !
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1024MAK
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Re: MTX new Replacement mainboard ??

Post by 1024MAK »

I would say that there is a third option.

Rather than using the existing 1980's MTX design, or using the MTX+ design, we design a MTX board using a Z80 running at the same speed as a original MTX, but using one ROM chip instead of two or three, using one 128k byte SRAM chip instead of eight DRAM chips. Using one 32k byte SRAM for the video RAM instead of eight DRAM chips.

Only one +5V supply input would be needed, as a small switching regulator could convert the +5V to the -5V needed by the tape and sound circuits. No need for a +12V supply. So no need for any of the regulator circuitry from the existing design.

Unless there is a desperate need, no need to include a TV Modulator either. If we reuse the video board, we would need a +5V to +12V converter, but this is not a problem. Do we stick with the original video processor chip?

For ease of design, some of the 74LS glue logic could go in a CPLD, or we could keep the 74LS glue, but optimise it somewhat. After all, how many people actually use the on board parallel "user" port? The printer port could be optional as well (although from time to time, I do use this, just not always for printing :mrgreen: ).

Any other thoughts?

Mark
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Re: MTX new Replacement mainboard ??

Post by Martin A »

How easy is it to fit the VDP with SRAM rather than DRAM ?

The only article I've seen was really only hinting at a possible solution, and made it appear a fairly complex job, thanks to the rather weird way the VDP accesses memory.

Replacing the 3 voltage 6116 with single voltage 6164 is easy if you're just after simplifying the power supply.

One of the MTXPlus+ test boards has 7x 8k rom images in a single flash chip as well as 128k sram, with all the address logic in a single 16v8 SPLD for a 3 chip solution. The usual Memotech setup would have been 20 or more devices (2 roms, the PAL, and 8 DRAM on the MTX512 main board, 8 DRAM, 3 LS chips and another PAL on the 64k ram expansion board)

That setup still needs a second 16v8 and 74xx273 for the page port and the minimal I/O decode that board supplies.

I believe Dave has pretty much all the logic in a single CPLD on his board.
lezanderson
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Re: MTX new Replacement mainboard ??

Post by lezanderson »

Reverse Engineering the Original board using schematics from the original manual will be very difficult and fraught with problems. Probably the best option is to design a NEW motherboard that will fit inside an MTX case and use original MTX PSU.

Specification
Speed : (4/8MHz software switchable)
board size same as original motherboard with same connectors for compatibility.

Z84C0008PEC Z80 CPU 8MHz
Z84C3008PEC Z80 CTC
Z84C4008PEC Z80 SIO
EPM7032SLC44 (Memory mapper CPLD) PLCC44
128K SRAM DIP32
128K SRAM DIP32
512K NVSRAM DIP32
64K EEPROM DIP28
EPM7128SLC84 CPLD (I/O Mapper + keyboard, printer+ Joystick)
8255 PIO for CF card or hard-drive
DS12C887/DS12887 RTC clock calendar function DIP
MAX232 for SIO RS232
MAX705 for reset Function
SN76489 sound Chip PSG DIP16
YMZ284 PSG (AY-3-8910 compatible) DIP16
Power supplier by original PSU

** giving 768K RAM + 64K ROM
** The EPM7128 will interface to printer MTX keyboard, Joystick etc

Two sound Chips SN76489+YMZ284 for improved sound functions


********************************
Video
V9958
D41464-10 6pcs (192K Total)
CXA1645 PAL/NTSC + composite output

or

F18A for VGA output connected to a DIP40 socket on motherboard.

*************************************


This will give a chip count of between 20-30 ICs about half the original MTX motherboard with Much, Much improved functionality and could (possibly be put on a double sided board ?) This would push the original design to it's MAX RAM,ROM etc yet being considerably cheaper and probably easier to design than reverse engineering the original schematics to make a COPY of the original motherboard.
Last edited by lezanderson on 15 Jul 2015 10:56, edited 1 time in total.
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Crazyboss
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Re: MTX new Replacement mainboard ??

Post by Crazyboss »

I was not thinking about make a new copy of the mainboard, but a 100% compatible mainboard, but as mark said do we need the tv modulator?

I some way I wanted to know if it is possible with modern hardware, to create a compatible mainboard, which have all the original features, uses less power and maybe a few extra features could be added.

My idea is this mainboard should still work 100% as the original with the possibility to use the original add-ons like FDX, SDX, RS232, REMEMORIZER and MEGAROM.

Maybe this new design could have vga/hdmi/component out ? and maybe its possible to have a chip can handle the more functions, like combine more of the common ic's like: Z80, CTC, VDP, the SN-Audiochip etc.

The Latest (1990 or so) MSX computer (called MSX TURBO-R)), was 100% compatible with MSX1,2,2+ the turbo-r was actually a MSX2+ with a few extra things like PCM sample option and a R800 risc chip emulated the Z80 in someway. Actually it dont have any TAPE port! This computer still had the original MSX2+ VDP Yamaha 9958.

But did not have the original AY (PSG) chips in stead it have one or more custom chips "emulated" those function, no Z80 ether, but a R800 risc processor programmed to run z80 in 3.58mhz or 7mhz turbo-mode :)

Not sure if the FMPAC sound was in the custom chips or had real hardware?

But some kind of design like this could be redone for the MTX :) but it might take years, and I do believe the end product will still be around £100?
//CLAUS - Webmaster at www.mtxworld.dk
lezanderson
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Re: MTX new Replacement mainboard ??

Post by lezanderson »

Looks like Clause is volunteering to design a NEW MTX motherboard ? This will be quite a tricky job , as the original board had some 60 or so ICs on the board, Several versions were made, NTSC, PAL etc and was a multilayer board ? Not sure whether you can still source all the ICs, there's probably more questions than answers regarding this project. Well it'll keep him out of mischief for some time !
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Crazyboss
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Re: MTX new Replacement mainboard ??

Post by Crazyboss »

it have been a while :)

Been thinking about it again, what i was hoping, that someone who knows how to use the electronic design programs could create a "new" mtx mainboard, with sockets for cpu/vdp etc. So when you find a "dead" mtx, its just put in the new board, and put the common ics in there.

There could also be an option where Andys Rememorizer could be build into the board, but then there need to be a way to fit the sd card. Maybe if the RF modulator was removed there could be space enough for a SD card, or maybe the paralla port could be removed.

I am not thinking about any to upgraded mainboard, I would still like it to be compatible with the old hardware if possible, like the FDX box.

Not complete sure how many mtxs i currently have, and how many works :)
//CLAUS - Webmaster at www.mtxworld.dk
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Dave
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Re: MTX new Replacement mainboard ??

Post by Dave »

Fixing the broken MTXs would be easier and cheaper.

If you pay for postage and parts I (or, I'm sure, any number of us) could probably do it for you.
If the bare board was posted, I don't imagine it would be too expensive.

A replacement board should have different power supply arrangements from the original to get away from needing the dedicated MTX PSU. Otherwise, replacing a failed PSU would still be problematic. A direct replacement for a failed MTX PSU is pretty hard (impossible) to find and Mark's replacement option is not cheap. Better to use a DC only supply for any replacement board design. Ignoring RS232 and fitting different VRAMs it could be 5VDC only. So, not a direct replacement, but a far better option. I **might** get around to that at some point, but far too many other projects on the go at the moment

regards
Dave
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1024MAK
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Re: MTX new Replacement mainboard ??

Post by 1024MAK »

Repair and servicing of existing MTX boards is the most practical solution.

This then allows 5V RAM chips to replace the triple supply 4116 DRAM chips.

A straightforward DC/DC converter can then generate the +5V supply. If RS232 is required, another DC/DC converter can generate the -12V supply. Then the board can be powered off a +12V DC PSU.

My replacement PSU was intended to be used only if the owner wanted no changes to the MTX main board.

Designing a new main board is indeed possible. But due to it's complexity and size, it's likely to be expensive and will probably need several revisions before a version that does not need modification wires is produced, making it expensive :o

Mark
Last edited by 1024MAK on 07 Jul 2018 12:03, edited 1 time in total.
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