Apple III repair / missing logic PROM device

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Apple III repair / missing logic PROM device

Hi,

I'm currently resurrecting an Apple III. It's a machine which was broken and (nearly) dumped almost 30 years ago. I litterally found and pulled it from a hardware garbage pile when I was young. Back then, I was too young and too inexperienced though, so I could not get it to work. It's been sitting in a box in my basement ever since - travelling through time (and space) :-) - awaiting a happy day, where it would finally be saved... But I never found the time and had almost forgotten about it. Well, until now that is... I had a working Apple IIe ever since (my first computer). Thought it would be nice if the Apple III finally joined.

The Apple III Service Manual is absolutely great - certainly one of the best and most detailed service manuals I have ever seen. And the mainboard is so incredibly servicable - almost all the ICs are in sockets and easy to swap.

I already managed to fix two major issues:

  • First, it would not boot at all (a "NO BOOT" issue as described in the service manual, where the machine would just do nothing at all when switched on - and the screen stays blank). I've traced this issue to D13/SN74374, the driver for the memory board's CAS signals. Without this chip, no memory cell will work - so the boot ROM locks up immediately when running its initial zero-page memory test (which is about the first thing the ROM does, before even enabling the video output). According to the manual D13 was the most likely cause for the "NO BOOT" issue anyway. The manual was certainly right in this case. With a fresh replacement 74374, it finally booted - for the very first time after almost 30 years of sleep. Yay! :-)
  • Then, it always entered "diagnostics mode". The diagnostics did not show any errors, but the keyboard didn't work. Luckily, this was also easy to fix. Traced it to a broken H12/74257 - another standard TTL device. Replaced the chip - and the keyboard works again. Now it no longer enters "diagnostics mode" when booting, and defaults to boot from disk instead. Yay again!

But unfortunately, it's still not fully fixed. Currently I'm stuck with issue #3:

  • The internal disk drive isn't working. When booting, the machine enables and resets the drive, but aborts immediately, writes "RETRY" and endlessly repeats this all over. I have investigated this and currently believe the issue to be K11/P6A (the PROM with part number 341-0028). I can trace the disk's read data signal all the way to K11 (entering A4/pin 5 at K11). But it's output pin D2 (pin8) is always low. This is where the decoded data signal should be fed to the 8bit shift register (K10). Since it's output signal is always low, the boot ROM does not see any changing disk I/O data - hence believes the drive's motor wasn't even spinning.

Seems conclusive to me - but now I'm stuck: that's a custom Apple logic device. And I've been unable to find any offers on ebay or elsewhere.

 

Long story short: any suggestions where to find a suitable replacement/spare part for the Apple III's disk drive's logic PROM (341-0028)? Also, I noticed the service manual contains detailed descriptions of all logic PROM devices (essentially these devices seem to be simple 256 x 8bit PROMs with containing custom programming). However, the manual has no such description of the logic contained in this specific device for the "disk conditioning" logic.

 

Any suggestions/hints are highly welcome...

T.

 

PS: Sorry, somehow I have created a duplicate/empty forum topic. Not sure what I did there...

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341-0028 PROM

The 341-0028 is the disk drive state machine PROM and is also used on the Apple II Disk Controller Boards

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Excellent hint

wsander wrote:

The 341-0028 is the disk drive state machine PROM and is also used on the Apple II Disk Controller Boards

 

Oh, common components - I haven't thought about that. That indeed could simplify the hunt for a replacement. Excellent hint, thank you!

I checked the controller in my Apple //e. It has an 341-0128. Probably a newer version of the same device in the //e.

Meanwhile I plugged my 341-0028 into an (E)PROM reader and dumped its contents. Unfortunately this confirmed that one output (D2) is stuck at 0. So it's definitely broken. I'll compare this to my //e's device - and see how else they differ.

T.

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MacFly wrote:Oh, common

MacFly wrote:

Oh, common components - I haven't thought about that.

See also: https://kb.pocnet.net/wiki/Apple_Chip-Bezeichnungen

Comments, corrections and additions are welcome :-)

 

Regards

Ralf

 

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RalfK wrote:See also: https:/

RalfK wrote:

See also: https://kb.pocnet.net/wiki/Apple_Chip-Bezeichnungen

Comments, corrections and additions are welcome :-)

 

Nice. You can add the "341-0128-A" to this list. It's a "16 Sector drive controller P6A ROM" as found in a disk I/O controller card of an Apple //e (see image). It probably very similar to the 341-0028 device, since the schematics of this I/O controller and the onboard logic of the Apple III only differ negligibly.

 

Has anyone collected and archived the contents of these logic devices? These are 63xx PROM devices (most of them are of the 6309 type, so 256x8bit PROMs): https://archive.org/details/6309PROM

Apparently some distributors still have plenty of those on stock, selling them for a few bucks each. So it would still be possible to make fresh replacements, without needing to sacrifice a donor machine. A special programmer is needed though - the standard EPROM programmers cannot program those (though some will be able to read and dump their content, which is trivial).

I compared the content of my 341-0028 and 341-0128 PROM devices. They are quite similar - but still differ in many bits. Since I already know for sure that my 341-0028 device has a defect (at least one output is stuck), that's not too surprising though.

 

T.

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MacFly wrote:You can add the

MacFly wrote:

You can add the "341-0128-A" to this list.

Done. Thanks.

 

MacFly wrote:

It's a "16 Sector drive controller P6A ROM" as found in a disk I/O controller card of an Apple //e (see image). It probably very similar to the 341-0028 device, since the schematics of this I/O controller and the onboard logic of the Apple III only differ negligibly.

P5 contains the firmware code of the disk controller. This code is well known. P6 is a kind of state machine?

 

MacFly wrote:

I compared the content of my 341-0028 and 341-0128 PROM devices. They are quite similar - but still differ in many bits. Since I already know for sure that my 341-0028 device has a defect (at least one output is stuck), that's not too surprising though.

I found some photos of the early disk controller cards: the controller with the two 20pin headers has the 341-0028-01/-A, the Duo Disk controller has the 341-0128-A. Is that always true?

 

Regards

Ralf

 

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RalfK wrote:P5 contains the

RalfK wrote:

P5 contains the firmware code of the disk controller. This code is well known. P6 is a kind of state machine?

The P6 is a tiny ROM (256x8bit) and as such it is stateless itself. However, some of its address lines (inputs) and data lines (outputs) are connected to flipflops (74LS174), so together they implement a state machine. The bit patterns programmed into the P6 PROM control the state machine's logic. They also control the 8-bit shift register, which converts the parallel data to a serial bit stream (when writing to disk), or converts serial disk data to parallel (when reading the disk).

 

RalfK wrote:

I found some photos of the early disk controller cards: the controller with the two 20pin headers has the 341-0028-01/-A, the Duo Disk controller has the 341-0128-A. Is that always true?

 

Mine is also a duo disk controller. However, the P6 PROM doesn't know anything the number of connectors. Technically, it could be used in both cases. I guess Apple had improved something about the bit timing or encoding, hence created a new part number for the updated PROM.

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The P5A and P6A PROMs are a

The P5A and P6A PROMs are a 6309 style 256x8 bipolar PROM.  That's the MMI part number, which was commonly used by Apple.  Compatible chips were made by a bunch of companies like Signetics, Harris, TI, etc.

 

A good reference for these chips is here:

 

https://www.mikesarcade.com/cgi-bin/spies.cgi?action=url&type=info&page=PromRef.txt

 

Unfortunately these chips haven't been made in a long time and new-old stock of blank parts has pretty much dried up.  Also programmers which can program these parts aren't common any more and often only are supported by ancient software, computers or interfaces that modern computers can't deal with easily.

 

However, the suggestion that the parts are used on other Apple products is a good one.  Buy an Apple Disk ][ controller card or one of the later Apple 5.25" Controller Cards off ebay or wherver and pull the part off of it.  Those cards are super common and fairly cheap.  Especially if you can find one that has issues like the ribbon cable pulled off the later cards with the DB19 connector or the pins bent on the IDC20 connectors on the older ones.  I've seen those sell for as cheap as about $10.  And since you only need the one chip, not an issue.

 

 

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