MEK6800D2 restoration - test-drb@ccmp.vtda.org

23 Nov 2010

/
/>  I ought to have pointed out that most times doing what i do wastes a
...
   little time, and in fact it owuld heen fine just to
plug the board in and
  power up. But this is amply compensted for by the time saved when thigns
  go very wrong. 
Yeah and when you've gone through all the steps, learned what all the chips
are for, plugged them in one by one and finally the whole thing does actually
work, the satisfaction is much bigger.

...
   Do I conclude from this that the first byte read ---
in fact all bytes
  read -- are always 0? 
Exactly.  And I just figured that they all are framing errors. Here's my test
program that should exit when it reads a byte that has no framing error:

	LDAA #%00010000 ; like in jbug (8bit, np, 2stop, no divide)
	STAA $8008 ; acia control reg
READ	LDAA $8008 ; acia status reg
	ASRA
	BCC READ  ; branch unless data ready
       	BITA #%00001000 ; check framing error
	BNE READ ; branch if framing error
         SWI

It never exists no matter what audio file.  The only thing that does work is
that the data ready bit stays cleared  as long as there is no audio at all.
So it at least reacts to the fact that there is audio or not.

...
   I assume this ACIA is a 6850. What does the data
input do when you play
  the audio file? Where does the Rx Clock signal come from, and is it correct? 
Yes, it's a 6850.  I measured with a volt meter, I don't have a scope (yet).
RxD and RxC are high when no audio and they both drop to around 2V when audio is
supplied, so I guess there's at least some signal.  Also RTS is low as it should.

Something weird that I don't understand is that simply adjusting the audio volume,
the apparent voltage measured on the RxD changes more or less proportionally...
this is supposed to be digital and FM ?

Wim.