Re: Two new PRGs

From: Ethan Dicks (
Date: 1998-02-17 09:50:10

Olaf writes: 
> The "fast print poke" is the same as your killer poke, I think.


> I suppose the problem arises when two outputs are connected to
> each other and try to drive the line to different levels. Or so
> I'm told - unfortunately I'm not much of a hardware techie.

The circuit in question is called a totem-pole input.  The vertical
blanking signal is routed to a pin on a PIA (I don't have the info
right here to reference).  IIRC, the fast print poke toggles the
data direction register for the input in question, making it an output.
On older hardware, there is no problem, and the PRINT code in the
ROMs checks the bit which is now always true, and doesn't wait for
the VBI, making prints faster.  On the oldest hardware, this causes
lots of snow.  On middling-old hardware (8" 40-column machines), it
looks fine.  On some class of newer boards, I am told that it can
smoke your monitor.  

Without a schematic in front of me, I have to recall this from very
dusty memories, but it works something like this: the input pin to
the PIA is attached to the VBI circuit by way of a pair of transistors
arranged in a "totem pole" fashion.  This name arose because the
visual representation  of these two transistors, one above the other,
must have resembled a native American totem pole.  When the former input
is made an output, the PIA doesn't care and now returns a 1 to any code
that read it.  What cares is the transistors - the output signal from
the PIA is now driving a transistor instead of being driven by it.  This
is bad because in a totem pole arrangement, the collector of one 
transistor is attached to Vcc (+5V), the other, to GND.  If a design
engineer never expects this arrangement to be driven, there are no
current limiting resistors in the path, as is true for the PETs in
question.  The end result of the Killer Poke is to attach +5V to GND
through a couple of transistors which can't take the current, and POOF!

I have a sheaf of PET schematics, but they are such bad copies that I don't
know how well they will scan in.  I think it's time to try.  If I had all
the time in the world, I'd convert them to OrCAD format, but that's a *lot*
of work.  Any PET hackers out there use OrCAD?


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