[Coco] DS-69B Reverse Engineering

[Joel Ewy]

Years ago I disassembled the little assembly routine loaded by the
  RANDACB.BAS program for the DS-69 Digisector.  I also cut open the glued
  plastic case of my Digisector, got an ohm meter in there and scribbled up a
  partial schematic.  I even have vague memories of having unsocketed the GALs,
  stuck them in my programmer, and learned that the security fuses hadn't been
  blown, but a cursory search hasn't yet turned up the fuse maps.
  
  I have a couple pages of semi-commented disassembly and notes that floated to
  the top of the piles recently, and my interest in the DS-69 is at a high ebb again.
  
  There's a mysterious section of the code that doesn't make any sense to me and
  I wonder whether I just don't get it (entirely plausible) or if the copy of the program
  I disassembled 15 years ago was already corrupt.  (Mind you, this is the 
  RANDACB program, not C-SEE, the main program.)
  
  I just tried making a new copy from the original floppy.  It took me 3 tries to get 
  DECB to make a backup without I/O errors.  I have little faith that what I got is a 
  faithful replica of the original.  Since RANDACB is only used by the sample 
  BASIC programs (which I never really used back in the day), and not the regular
  digitizer software, my copy of RANDACB may have been bad all along, for all I 
  know.  I chose to disassemble it instead of C-SEE because it is much simpler,
  smaller, and easier to understand than C-SEE, and is really only involved with
  running the digitizer and communicating with a BASIC program.
  
  I might add that I recently tried using the SLOWPIC.BAS program to make a
  64-level image that I could massage into something useful with netpbm tools.  
  It did create an image, but it was peculiarly distorted in that every 16 columns it 
  would appear to back up and re-digitize the previous 16 columns.  If anyone's 
  interested in seeing what I mean I can email you a picture.  The type of error it is
  would seem to me to be coming from a bug in the BASIC program instead of the
  assembly routine.  But I haven't yet ruled out that the mysterious code section
  might be causing this problem.
  
  1.  Does anybody have a working DS-69B setup?  Any interest in aiding a project
  to understand how the thing works with the aim of writing a driver for (Nitr)OS-9?
  I'd like to compare my version of RANDACB with another presumed-good copy.
  
  2.  Does anybody have any bright ideas about what the following code snippet
  could possibly be doing?  It may be corrupt.  It may be intentionally obfuscated.
  Maybe there's some other place in the code that I haven't noticed yet that
  modifies this section so that it makes sense.  I don't think that's likely, but it 
  certainly doesn't make much sense to me as it is.
  
  The code segment is part of the routine that reads brightness values of a pixel.
  I think we are doing successive approximations, at least partially in software,
  which is why it takes about 4 hours to digitize a 256x256x6-bit image with this
  software, unless you use the high-speed POKE.
  
  ...
  7F9D    8607    8:     LDA   #7
  7F9F    B7FF71       STA    $FF71   -> PIA register on the DS-69B
  7FA2    10AC9DDEF4    CMPY    ( 57076,PC)   Mysterious section begins
                                   We  haven't been using Y, and don't seem to later
  7FA7    12                    NOP
  7FA8    12                    NOP
  7FA9    12                    NOP
  7FAA   1012            ????   The disassembler chokes on this, and it does
                                      appear to be an illegal opcode (on the 6809).
  7FAC   E661            LDB   1,S   Now we start making sense again
  7FAE    B6FF22        LDA    $FF22   I think we're enabling an IRQ here
  7FB1    B6FF70        LDA    $FF70    Ditto in the DS-69's PIA
  7FB4   13                SYNC   Here we wait for the IRQ.  This  makes sense
  7FB5    E484              ANDB    X+0
  7FB7    ...
  
  3.  If anyone is interested I can email the rest of the code and comments and 
  scans of my provisional schematics.
  
  JCE