Memory problem with Nms 8280.

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By Wild_Penguin

Champion (478)

Wild_Penguin's picture

04-10-2018, 16:23

Hi x1pepe,

1bit mapper = 32kb(ytes), 2bit = 64kb, 3bit=128 and so forth....

You have back annotation = 8bits = 4MB (megabit) mapper??? I take it you have some mapper in slot 1, does it really have that large a mapper? (Playsoniq?)

In any case, testram can tell us it can only find a 2-bit mapper at slot 3.2. See 8280 SM, page 25:
http://msx.hansotten.com/manuals-and-guides/

I've been playing around with the memory mapper of a 8255 previously, so I know how it works (well, roughly). The LS670 has the actual register (IC131 = 74LS125 is for back annotation, which is not required per MSX spec, however these machines do have it). D0, D1 and D2 are the inputs (pins 1, 2 and 15), and EA14, EA15 and CAS2/E are the outputs (actually, it's a 4x4bit register, out of which 4x1bit is not in use, but in your case only 4x2bit is working; see http://pdf.datasheetcatalog.com/datasheets/70/375685_DS.pdf in case you're interested).

The bottom line: you should have 3 bits working on that mapper chip. Something is wrong; I'd scrutinize the traces for scratches on the PCB and/or short circuits (unlikely) here (IC149, IC131, IC150 and traces between them). If you can not find any thing obviously wrong, you could debug it with probe or oscilloscope (I've never done that). Either the LS670 or some other part of the mapper circuit is broken (IC131, IC149). It's much more safer to test these chips than begin desoldering them blindly (removing possibly working chips and possibly damaging your MB in the process is not a good idea).

After correctly diagnosing the problem, it's much safer to replace a chip (since it is less likely to do additional damage!). However, my guess it is quite likely this LS670 is broken, since if it was the 157 (IC150), the MSX should not boot at all without external memory mapper (?). But, I could be wrong!

By mohai

Paladin (738)

mohai's picture

07-10-2018, 13:50

mohai wrote:

This model has 2 banks of 64 KB each one.
From the mapper chip, upper bit is mixed in a simple way with RAM chip select from the engine in order to select one or another bank.

If you can get into BASIC, you should test if the mapper is working fine or not.
Try:

? INP(&HFC)
? inp(&HFD)
? inp(&HFE)
? inp(&HFF)

should show 227,226,225,224

Sorry for my typos.
For an MSX with 128K mapper, values should be 251,250,249 and 248.

As for the values read in your MSX, it seems that the high bit in the mapper is always in a high state ( =1 ), that is the reason why the tests show that there are only 64 KB in the high bank.
The fault is easy to trace, as Wild_Penguin says. Only one signal needs to be traced.
First, take the service manual and look for the 74LS670 chip and the chips connected to it.
Chip 74LS670 can be broken, any chip connected to it or maybe just a bad solder. Visual inspection and re-soldering can sometimes be a solution.
A cheap probe will help to diagnose too.

By x1pepe

Resident (52)

x1pepe's picture

09-10-2018, 01:08

Thanks folks! I will try this weekend. Smile

By x1pepe

Resident (52)

x1pepe's picture

10-10-2018, 18:15

Wild_Penguin wrote:

Hi x1pepe,

1bit mapper = 32kb(ytes), 2bit = 64kb, 3bit=128 and so forth....

You have back annotation = 8bits = 4MB (megabit) mapper??? I take it you have some mapper in slot 1, does it really have that large a mapper? (Playsoniq?)

I put it "Flashjacks" cartridge with MAPTEST in SD Card.

By x1pepe

Resident (52)

x1pepe's picture

10-10-2018, 18:22

Thanks to all with yours suggestions, I could do it. :D
There was some scratch near IC131 than I can fix it resoldering.

https://drive.google.com/file/d/1x2iR9d12mBvdCL2sP1Wh4WMEJdI...

By Wild_Penguin

Champion (478)

Wild_Penguin's picture

10-10-2018, 19:03

x1pepe wrote:

Thanks to all with yours suggestions, I could do it. Big smile
There was some scratch near IC131 than I can fix it resoldering.

So you got it working? Congratulations!

(yes, Flashjacks explains the back annotation and we can see the mapper at maptest.com)

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