Apple II+ 48k 6502 and/or rom issue

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Apple II+ 48k 6502 and/or rom issue

I have a 48k apple II+ with no expansion cards, and whenever I try to execute certain AppleSoft basic functions (HCOLOR, COLOR, PLOT, HPLOT, IF, FOR, LEFT$, just to name a few) it spits back a memory address at EC46 with different values depending on the function it didn't like. In monitor mode, when I run Ctrl+E to get the 6502 registers, it has the same values every time.

I'm not an expert in electronics, but from the research I have done and my limited programming knowledge this leads me to believe the problem is either in ROM or the 6502 itself, more likely the 6502 due to some AppleSoft commands working. I haven't tried cleaning the chips yet, but I was wondering if anybody who is more experienced could help diagnose the problem further, so I can narrow down the culprit. Beyond that, if anybody has any suggestions to fix, replace, or repair the 6502 or ROM, and where to get those parts, that would be helpful as well.

Earlier today I took it apart and cleaned it to fix the PSU, so I think it's probably not an issue of corrosion or the like, but I still need to double check. I am the second owner of it, so wear and tear is on the lower side.

attached is a video of it giving me the error code from AppleSoft, thank you to anyone who can help!

 

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Sounds more like a ROM issue.

Sounds more like a ROM issue. If you have a ROM card (or Language Card) you could use that to substitute for the motherboard ROM to prove that. Or download a binary image of the E8 ROM and compare it to yours.  There are also many diagnostic disks that you can run that test the ROM.  

Replacing the ROM would require finding an oem ROM or using an EPROM with adapter. There's also the ROMX which we sell through ReActiveMicro.com. 

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I agree that it sounds more

I agree that it sounds more like ROM.  If it was the 6502 it seems like it would affect a lot more than just graphics.

 

Testing the ROM and/or comparing to a known good image is a good start.

 

 

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Do bootable assembly language

Do bootable assembly language-written disks run properly?  Like games, for instance?

Something that runs in machine code and bypasses ROM but requires most of the RAM?

That would go a long way to figuring out if ROM or RAM is the culprit.  

Have you tried running any diagnostics like APTEST or Apple Cillin? 

Your post says you have no expansion cards...you can still load software (games) via the cassette ports and a smartphone through the website http://asciiexpress.net/gameserver/

 

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List the area

Do a CALL -151 (which, even if it breaks, will get you into the monitor, heh), followed by EC40L, and compare the first few lines of output with the following:

 

]CALL-151

*EC40L

EC40-   85 9E       STA   $9E
EC42-   85 9F       STA   $9F
EC44-   85 A0       STA   $A0
EC46-   85 A1       STA   $A1
EC48-   A8          TAY
EC49-   60          RTS

 

If it doesn't match that (looks to me like EC44 is liable to be a 00/BRK), then your ROM is bad.

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Thanks - I tried this and you

Thanks - I tried this and you were right, $EC44 was a brk and a few others didn't match as well

if I'm going to replace the rom chip is there a good way or place to get it? It seems to be relatively cheap but not as easy to find.

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 LennyStudios wrote:Thanks -

 

LennyStudios wrote:

Thanks - I tried this and you were right, $EC44 was a brk and a few others didn't match as well

if I'm going to replace the rom chip is there a good way or place to get it? It seems to be relatively cheap but not as easy to find.

Finding an OEM replacement for a single ROM is going to be hard. Many sell EPROM substitutes but that requires an adapter socket. You might be better off replacing ALL of the ROMs with one of the ROM boards such as the MultiROM or ROMX (which I helped design and is available for under $50).

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While I still want to look

While I still want to look into other solutions as well and do some reaserch (ideally I can find a replacement for just that chip but as you said it would be difficult), but ROMX looks fairly simple to implement and much easier to get.

 

at least now that I know the issue I can fix it and source what I need -

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ROMs in Apple II can be replaced by common EPROMs.

In post #8, 'LennyStudios' wrote:

 

" While I still want to look into other solutions as well and do some reaserch (ideally I can find a replacement for just that chip but as you said it would be difficult), but ROMX looks fairly simple to implement and much easier to get. "

 

Uncle Bernie says:

 

All ROMs in any Apple II have almost a standard 27XX EPROM footprint, with the exception of a few pins which are mask programmable chip selects. Early Apple II had 2kByte ROMs, which could be replaced by 2716 EPROM, but you could also use 2732 EPROMs, which are easier to get.  You need an EPROM programmer to program them. The ROM images can be found on Asimov.

 

The only 'hardware' challenge is to replace the up to three mask programmable chip selects on the ROMs with a TTL address decoder. This could be done with a single 74LS138 piggybacked on some other TTL. You would bend up the pins on the EPROM which sit in the three programmable chip select locations and wire them to the appropriate decoder outputs.

 

Even easier would be to pull the whole upper 8kBytes of ROMs and put a type 2764 EPROM in one of the sockets. The four upper pins of the 2764 would hang in the air, but they still can be flight wired as needed. Providing a suitable chip select for this sole EPROM would be less effort.

 

But all of this requires -

 

a) some fairly low level understanding of digital electronics (what are EPROMs ? What are chip selects ? What is an address decoder ?)

b) an Apple II schematic

c) proper tools (soldering iron , EPROM programmer, if EPROM not blank, an EPROM eraser, too)

d) some thinking on how to wire everything up

 

No everybody has these tools or skills. But I would repair an Apple with bad ROMs using EPROMs, as I have all the tools and skills.

 

However, before shelling out money on anything, try if pulling the ROM from the socket and putting it back helps. Some ROMs I saw in Apple had badly tarnished pins, and the sockets used by Apple at the time being were cheap crap, too. So maybe it's just a contact problem. Which you should rule out before spending money.

 

- Uncle Bernie

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As usual Uncle Bernie brings

As usual Uncle Bernie brings up a good point.  Bad contacts due to dirty or corroded chip pins and/or sockets could cause a ROM to appear bad.  Also what he says about sockets failing is also true.   It would be unfortunate to do all the work to get another ROM or an EPROM substitute and then find out the original chip is OK but the socket needs to be replaced.

 

In my case I have multiple ][+ machines so I could try a different and known good ROM chip of the same programming  in that socket and see if the machines runs correctly.  However I suspect you don't have an extra one to try...  But you do have the other ROMs.  While you won't read the correct data for the location you are looking at, if you swap one of the other ROMs into that socket it should display the correct data for the other location.  You just need to know what data to expect, but you can check that from the monitor before you swap the chip over.

 

I hope that all makes sense...  it's a little bit of an unuaul suggestion I guess.  But it should work to test to rule out the socket being bad.

 

 

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That all makes sense - I didn

That all makes sense - I didn't really think about the socket so I'm going to go ahead and check that soon, if not I found someone who is selling EPROMs with AppleSoft already written (and a few other Apple II ones as well), but I'm still going to look into it more before I buy.

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