MSX Turbo-R behavior with 1MB external mapper

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Par l_oliveira

Hero (532)

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14-02-2016, 15:28

One of the reasons why I like to use the 74LS138 to decode I/Os is that I can make a lot of I/O ports on the same board with just two of them. I can make any address up to six bits.

You could for example make a jumper matrix and have any I/O port value possible with the design just toggling which of the eight outputs on each of the two 74LS138s you connect your two input OR port to.

Par sd_snatcher

Prophet (3498)

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14-02-2016, 15:38

Agreed. It fits the job perfectly well. I love this IC too. And also the 74LS139 and the 74LS682, for similar reasons.

Par RetroTechie

Paragon (1563)

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14-02-2016, 23:17

Hehe, did a little searching & had some fun with KiCad:

For any regular MSX that would add 1 bit to each of the 4 mapper registers. So if the circuit that l_oliveira posted above works on Turbo-R, then this should too as it's a functional equivalent.

The 74LVC1G332's are one IC each per gate. But tiny, and should be easy to do a pcb layout with. When you also pick SMD versions for the '30 and '670 (SOIC) then you could squeeze it onto a board of say, 2x2 cm. And still hand-solder everything.

Par tvalenca

Paladin (747)

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18-02-2016, 16:51

Only to comment that l_oliveira expanded my A1GT to 1MB using this design 3 years ago and it's working fine since then.

Par sd_snatcher

Prophet (3498)

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18-02-2016, 17:13

Thanks for the report! Have you tested the following software? Do they run fine?

- Unknown Reality Demo
- Fray (Turbo-R version)

Par tvalenca

Paladin (747)

Portrait de tvalenca

18-02-2016, 17:43

sd_snatcher wrote:

Thanks for the report! Have you tested the following software? Do they run fine?

- Unknown Reality Demo

Yes. Testing with UR is probably a must when upgrading the mapper due to Moonblaster driver (it reads back mapper registers) and the high memory usage (it needs 256k on top of DOS2).

sd_snatcher wrote:

- Fray (Turbo-R version)

Unfortunately not, since I don't have it. Maybe we can talk about it on e-mail (but I'm not asking here for a copy hahaha Running Naked in a Field of Flowers )

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