Memory problem with Nms 8280.

Expert (86)

30-09-2018, 17:32

Hello everyone, I'm resurrecting an old 8280 that had been abandoned for 10 years.
I have a problem with the User Ram, since it detects 64k (instead of the 128k), I have put the TestRam program and it detects that:

Testing frist 32k ok.
Testing second 32k ok.

But of the third and fourth memory (64k that would be missing to reach the 128k), it says nothing.
Do you think I have the third and fourth broken memory, or the problem vienna back? ... broken track, do not receive power, Bios? ...

By mohai

Paladin (835)

30-09-2018, 19:15

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

Then try out &HFE,2 and check some ram address such as &H8000 and &HA000 with poke and peek commands.
Repeat with out &Hfe,3 , 4 , 5, 6 or 7

This way, you will chek if ALL the second bank of 64 KB is wrong or only the mapper chip.
Anyway, I would recommend to download the service manual and figure out where the problem is.

A cheap data logger can help you find if selection chips are working fine or not.

By x1pepe

Expert (86)

30-09-2018, 21:57

Thanks for your explaining. I will try tomorrow morning.

By x1pepe

Expert (86)

01-10-2018, 09:21

That's it I did.
? INP(&HFC)
255
? inp(&HFD)
254
? inp(&HFE)
253
? inp(&HFF)
252

Sorry I didn't understand the second part

By Wild_Penguin

Hero (542)

01-10-2018, 10:11

Mohain (as far as I understod) has the idea to check the RAM manually with poke and peek commands. Look at their documentation to try to figure out what is needed.

That is unnecessary IMO. Try another tool called testmap (testmap.com). It can and should be able to test the RAM and mapper circuit in more depth.

If I've understod it correctly, the mapper circuit is at least partially working. But you've only tested two bits of it (two more bits and the actual RAM beyond 64kb remains untested).

It can be tested and diagnosed in BASIC, but since testmap utility exists, it makes sense IMO only as an exercise to learn these BASIC command, or if you don't have DOS or testmap available.

By x1pepe

Expert (86)

01-10-2018, 13:12

With testram. com program :

Testing first 32k.
Testing second 32k.
(No failures. Press space)

... In total are 64k, but this machine comes with 128k, there 64k missing...

By Wild_Penguin

Hero (542)

01-10-2018, 16:00

x1pepe: You've already told that, that's why I suggested trying testmap.com. It will tell in more detail (than testram) about the mapper(s) on the computer, and also about the RAM, and if it finds any errors.

I can see it can be a bit difficult to find with Google, since it is not in the download DB here.

http://www.msxarchive.nl/pub/msx/utils/misc/tstmap42.pma

By x1pepe

Expert (86)

01-10-2018, 16:27

Sorry Wild_Penguin I didn't read well the app, I will try it again.

By x1pepe

Expert (86)

04-10-2018, 10:01