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Here is a photo of an "impossible" Apple-1 daugther card under development in my lab:



The riddle - and the challenge to y'all - is to answer 3 questions, by commenting in this thread:


Q1: Which kind of peripheral card is it ?


Q2: Why is it believed to be "impossible" (for the Apple-1) ?


Q3: What does the screen contents mean ?


If there are at least three answer posts, each with at least two of the three questions above correctly answered, I will post more information about this development. And everybody who did answer at least two of the three questions correctly, will enter a sweepstake from which one winner is drawn.


This winner might receive the first production version of this daughter card from me as a gift - but no guarantees are made that this will ever happen. So you may get nothing even if you "win". The reason for this disclaimer is that I don't know yet if it ever can be made to work well enough to be a viable "product" - I don't want any marginally working gadget on an already quirky host system. And the next obstacle is whether there ever will be enough interest among the worldwide Apple-1 crowd to justify any PCB layout effort.


(BTW, it's not meant to be a money maker ... I just sought another "mission impossible" to keep my skills with various CAD tools and digital circuit design techniques sharp. These mental skills are like muscles ... use them or lose them. A real danger for retired people ... I've seen once brilliant engineers turning into dummies within a few years after their retirement. And this was not due to Alzheimer or such. It was due to mental sloth and rotting their brain away in front of the TV. The human brain quickly adapts to the abysmal IQ level found in there. Be warned ! (No April fool's joke, this warning !)).


But now back to our riddle. See, I want to keep your brain healthy, too !


Here is a proposed format for your answers (with a few hints, as I want to be fair):


A1: It's a ______  ______ peripheral card.


A2: It's "impossible" because Apple-1 has _______ DRAM _______.


A3: The screen shot shows ______  ______ from _____  _____.


You see ... very easy ! ... just fill in the blanks !


Good luck solving the riddle !


Your "Uncle Bernie"

Last seen: 22 hours 4 min ago
Joined: Feb 25 2014 - 23:19
Posts: 62

Hello Uncle Bernie,


Two 20-pin connectors... A GAL  array... glue logic ... Could it be a Floppy Disk controller?

More clues would be good.


Also, theoretically almost anything is possible, even if it means building a whole new computer on a card and using the A1 as a terminal only...

Last seen: 1 hour 28 min ago
Joined: Feb 27 2021 - 18:59
Posts: 17
Hi Uncle Bernie, and thanks

Hi Uncle Bernie, and thanks for the brainteaser.

I think this is a Disk II interface card. The two, 20-pin ribbon cables are pretty distinctive, aren't they? And, the chips are consistent with that (a multivibrator for pulse width, a shift register for data in/out, some NAND gates for stepper control(?), an addressable latch for bit swizzling).

Something about the DRAM refresh circuit on the Apple I is supposed to make it impossible to do precise timing loops, as is used in the DOS RWTS and ProDOS READ_BLOCK routines.

As for the data on screen, I do not know, but it doesn't seem to be valid 6502 code. Is it a nibble decoding table?

Macintosh_nik's picture
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Joined: Jan 8 2021 - 05:18
Posts: 28
Hi Uncle Bernie!

I also think that this is an Apple Disk II Interface Card only for A1, you can see two stubs and the layout of the elements is very similar.


This is not possible because A1 has maximum 8kb memory and Apple Disk Drive II needs at least 16.


What is on the screen is not even a guess. But very much want to know!

Last seen: 1 day 8 hours ago
Joined: Apr 1 2020 - 16:46
Posts: 133
The riddle contest is open until 1st of May !

Hey - this riddle was not meant to be solved just on 1st April.


It was posted on 1st April because it's kind of a joke ... sort of ... because it is like the Apple-1 itself: it almost works ! Oh the irony ! So I used it as an April Fool's joke.


You still have a chance to win such a card. I just need to find out why the green light goes off indicating that the 6502 software and the various state machines got out of sync. This no bug in my design ... I do have a complete hardware / software co-simulation which proves that it has no such flaws.


In one of my latest Apple-1 builds the green light stays on for hours so it works perfectly there ... until I power cycle the monitor. On other Apple-1 I have built it dies earlier but sometimes lasts for several minutes. It's obviously a signal integrity problem. This is where the most elaborate system simulations meet the real world. Which is rotten, nasty and it stinks ! No "ideal" signals to be found anywhere. System simulations can't emulate all that nastyness and all the bouncy "grounds". But they sure can help weeding out fundamental design errors. For instance, I found a very rare case constellation that was not covered in my original design so one state machine had to be extended to cover that case, too. In total there are more than 17 million distinct state trajectories in this system. And only a few would have done the wrong thing under very specific circumstances. This is the kind of bug you can only find by exhaustive system simulation. Which in this case has 12000+ lines of code.


There is a chance to make it work ! So don't give up and continue posting answers !


I'll keep this riddle contest open until 1st of May. And then I will give you the correct answers.  

Last seen: 1 day 8 hours ago
Joined: Apr 1 2020 - 16:46
Posts: 133
Found the root cause of the erratic behaviour ...

...it's the lousy signal on the CL clock line. So it's no April Fool's joke anymore. It can be made to work as designed.

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