Apple Language Card, ROM replacement and /INHIBIT

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Apple Language Card, ROM replacement and /INHIBIT

Hi,

My ][ europlus original ROMs are dead, so I added a https://www.ebay.fr/itm/Apple-II-plus-and-europlus-ROM-replacement-board-replaces-D0-D8-E0-E8-F0-F8/283900202214 

 

 

My goal was to get 64K of RAM. I've been lucky to score an original Apple Language Card on ebay in France, this week-end (not very common here).

 

When booting a disk with the card, some games did work, but many others didn't (for instance Oregon Trail). Booting a DOS 3.3 disk would fail to show the BASIC prompt.

 

I got a clue from a game that went from complaining that I had no 64K extension (before the card) to complaining that I didn't have 64K OR I had no BASIC ROM installed.

 

When booting and RESEting to the monitor I could see that the E000 space contained garbage (every read were different), so I figured out that the Apple Language Card was in some sense re-enabling the empty ROM slots of the motherboard (ie: I would get the Autostart ROM from the card, and nothing else).

 

Looking at schematics, I suspected that the /INHIBIT line from the card was responsible for this remapping.

 

I added a small piece of sticker on pin 32 of the card, and re-inserted it. Bingo, everything seems to work fine, and I have 64K of RAM!

 

(See sticker on pin 32, 7th from the top in the picture).

 

Q1: is this a common issue? Would another way of replacing the ROMs not be subjected to this problem?

 

Q2: are there better ways than a sticker on one of the card pins? I am worried that it may have long term impacts on the slot or the card itself.

 

Thx in advance

 

--fred

 

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Since you have the schematic

Since you have the schematic for the LAnguage Card you should be able disable whatever you want. But the card is meant to disable the motherboard ROM when he LC RAM is active. It does this through the INH line. By covering up that pin on the card you could get 64K of RAM (including the two D0 banks) but then you would have no BASIC on the motherboard.

 

Take a look at the LC softswitch locations to see if it is operating correctly. Sounds like it might be asserting INH when it should not.

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74LS09

The Language Card activates the INHibit signal (low) whenever its F8 Boot ROM is accessed - or one of its RAM banks is accessed. Just isolating the INH signal on the slot is not healthy. The card would still enable ROM or RAM access internally, so you get access conflicts on the data bus (Language card ROM/RAM and mainboard ROM will drive the bus at the same time). Even if the Language Card ROM had an identical content to your mainboard ROM, that's not what you want...

Looking at the schematic I'd say the most likely cause would be an issue with the 74LS09 (A4). That's the IC combining the card's two internal ROM and RAM access signals and drives the external INH line. An issue with this IC would be the easiest explanation for the effect you're seeing (i.e. the card is activating the external INH signal, but neither its internal ROM or RAM are actually enabled, so you see random data). The IC is easy to obtain and cheap.

You can also obtain other logic chips (74LS00, 74LS20, 74LS32) involved in the decoding of the address bus etc - just as cheap. Just less likely as I cannot see any easy explanation how another (single chip) defect could cause your issue (random data in E000 address range).

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You are awesome. I extracted

You are awesome. I extracted the 74LS09 and put it into my minipro. Sure enough, on of the 4 gates is dead.

 

I checked all the other 74LS of the board, and they are all correct. Unfortunately, I have no 74LS09 laying around, so it'll take me a few days to fix. I'll update the post.

 

Thanks again!

 

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It will work...

Hehe, excellent! :-) I'm sure it will work fine once you replaced the LS09. You had already narrowed it down to the faulty Inhibit signal. And when this IC was indeed defective, then that pretty conclusively explains what you observed.

That's the beauty with these machines. They are still simple enough, so you can make observations, look into the schematic - and pinpoint likely suspects/possible defects - all the way down to the individual signal/gate.

With modern devices, you're usually already lost at the point where you try to obtain a schematic at all. And even if you got one - things are usually too complex to still understand issues on a signal level.

Good luck - looking forward to your report of success... ;-)

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MacFly wrote:Hehe, excellent!
MacFly wrote:

Hehe, excellent! :-) I'm sure it will work fine once you replaced the LS09. You had already narrowed it down to the faulty Inhibit signal. And when this IC was indeed defective, then that pretty conclusively explains what you observed.

That's the beauty with these machines. They are still simple enough, so you can make observations, look into the schematic - and pinpoint likely suspects/p

Omnia est Verum. 

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@MacFly: You nailed it:

@MacFly: You nailed it: replaced the LS09, and everything works!

 

You are absolutely right about the beauty of those machines. As a software guy that is used to fully understand the software stack (well, until a few years ago, at least), revisiting those old machines and understanding what I missed at the time is a fun pastime.

 

 

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Très bien!

Très bien! Sehr schön...  Thanks for the update. Another happy Apple II. :-)

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