1024kb memory upgrade for Yamaha MSX2 YIS503

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By tvalenca

Paladin (728)

tvalenca's picture

18-06-2015, 17:27

I surely don't know about slot configurations o every MSX>1, but I don't think there's any with Sub-ROM on slot 0-x. I've been told once: "you will not want to have Sub-ROM on slot 0-x". (or, it was worse: "you CAN'T have Sub-ROM on slot 0-x". I can't remember right now). But I know that making inter-slot calls from BIOS on slot 0-0 to Sub-ROM on slot 0-1 will be a real pain in the ass, so I belive this does not happen on most MSX models.

So, it's more than appropriate for swap to a MSX2 Sub-ROM without any modifications. You can even mod it to dual-ROM, a single SPST or SPDT (or even I/O Mapped register on a free address) and you can boot either as MSX1 or MSX2.

By sd_snatcher

Prophet (3089)

sd_snatcher's picture

19-06-2015, 02:24

Grauw wrote:

Well, assuming the subrom has to be in slot 0-1... maybe it’s not a requirement. Also that ROM is in page 4000H afaik, I wonder if it’s also mapped to 0000H. My thought was, maybe the ROMs could be replaced with MSX2 ROMs without any hardware modification.

There's a way. First, take a look at this article. Pay attention to the sub-ROM mod.

But instead of cutting tracks, just:

1) Remove the old ROM and save it for when you want to undo the mod
2) Burn a new EPROM and insert it on the socket leaving the pins 20 and 22 lifted
3) Solder wires directly to the lifted pins 20 and 22 of the EPROM to the spots that article shows.

That's it. If you want to undo the upgrade someday, you just have to unsolder those two extra wires.

By Grauw

Ascended (8507)

Grauw's picture

19-06-2015, 20:17

It’s a good approach. My CX5MII has a different board though, marked CX-7 (it’s also the board that’s in the service manual), so that article, although referenced to me often, is not really so useful as the board layout seems quite different.

However since the CX-7 is an MSX2 I wonder how easy it will be. The YIS503IIIR service manual comes in handy. E.g. the wiring for adding an RTC can be lifted straight off page 71 and placed in the empty spots of the CX5MII’s board. Whether slot 0 or slot 3 is expanded can be determined by placing a diode (page 55), but page 33 says the YIS503IIIR’s subrom is also in slot 3-0 so that won’t be necessary. The CX5MII’s chip select for the subrom socket is fed by CS1 according its service manual so it won’t be a drop-in replacement. I can’t find what the YIS503IIIR does in stead though.

p.s. I’m missing page 30-31 from the YIS503IIR/IIIR service manual. Anyone got it? Also, if anyone has the YIS503IIIR schematics or the CX-7 service manual…

By Alexey

Guardian (2434)

Alexey's picture

27-06-2015, 23:12

o.geerdink wrote:

Alexy if you have it ill take it for my cx5m/ii 128.

Your board is ready to be shipped, please contact me.

Alexey

By palutti

Supporter (3)

palutti's picture

17-12-2016, 09:22

Hi Alexey,
do you still have available boards?

regards

By Alexey

Guardian (2434)

Alexey's picture

17-12-2016, 18:46

I have one board left. Please send me a PM via the forum. I also have the video board for this computer that has proper composite video output and s-video.

By Tabajara-Labs

Expert (93)

Tabajara-Labs's picture

22-03-2017, 11:00

Still have a board, Alexey?

By Alexey

Guardian (2434)

Alexey's picture

22-03-2017, 12:07

I will check when I am at home. But I think the last one was sold out some time ago...

By Alexey

Guardian (2434)

Alexey's picture

23-03-2017, 01:38

No, sorry, all boards are sold out.

By Tabajara-Labs

Expert (93)

Tabajara-Labs's picture

24-03-2017, 13:26

Mind sharing the desing, so I can make some?

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