[Coco] A hint for NitrOS-9, MPI, and "hard drive" PAK

[Gene Heskett]

On Thursday 19 December 2013 20:29:32 Robert Gault did opine:

> Gene,
> 
> You had better check the zeners D6 & D7 which are in that circuit. I
> can't look up the part number but from the Tandy RD3.9, I'd guess 3.9v
> zener diodes.
> 
> Since these are supposed to protect the circuit from over voltage, they
> ought to blow out before the SALT chip.
> 
> Robert

AIR from the schematic, those are on the rx data line.  I am way more 
concerned with having the full 12 volts from both rails on pins 15 and 16, 
thats about 4 volts above the usual +-8 the coco feeds in on those pins 
when its psu is functioning. I will check them however.

What is not stated anyplace is where the +5 volts for the tx out stage 
comes from.  I suspect pin 3, which in this instance _should_ be back 
driven from the 5 volt rail coming in from the AT style psu.  But its been 
close to 20 years since I did that, so it will bear being rechecked.  The 
logic swing is from -5 to +5, but I need to trace out which pin on the 4 
pin din is which before I can do a lot more.  What I see at the instant I 
checked last night was 2 pins sitting at 5 volts, one pin 8 or 10 
millivolts from ground, and one pin perhaps 200 millivolts from ground.

There is an exercising 3 liner in basic in the manual too, but my knee 
isn't up to messing with it tonight. I also bought 2 chips from the link 
Luis posted.  I already have a tube of 16 pin sockets.  Then the question 
is, can I find room under Tony's 2 meg ram kit for a heat sink so I don't 
blow another chip.  Could be "fun".  For some.  Me?  Its what I've been 
best at for at least 65 years now.  To the bad part troubleshooting.
> 
> --
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Cheers, Gene
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