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