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Author Topic: The Great Extendo  (Read 735 times)

stepleton

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Re: The Great Extendo
« Reply #15 on: August 22, 2020, 06:15:53 pm »

Digi-Key have some 56-place card-edge connectors, so I'll give those a whirl once the PCBs are ready to go.
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stepleton

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Re: The Great Extendo
« Reply #16 on: September 13, 2020, 04:17:20 pm »

The Mark 1 extendo is completed now, and it appears mostly to work (see attached image).

"Mostly" reflects how (in my 2/10 at least) the card fits in slots 1 and 2 but not slot 3, where it can't be inserted all the way. I'm not quite sure why this is. I don't use Slot 3 very much in my 2/10 --- the network card fits there, but that DMA test card I bought doesn't quite fit either. (This is an annoyance since Slot 3 is the best place to put a card that does DMA.) I think my own card is maybe about half a millimetre too wide and needs to be trimmed, but I'm not certain.

My card design also needs to be filed down about 0.5mm just in front of the card-edge connector.

Another aspect of "mostly" is that my RESET/ button is ill-conceived---I had assumed that I could use a pushbutton and a resistor to pull down RESET/ and reset the computer. Instead, it appears that (a) RESET/ can't be used that way, it's mostly an input that tells an expansion card to reset itself, and (b) if I wanted to wire RESET/ somewhere, I ought to have wired it to pull RESET/ up to +5VStdby, which can turn on the computer! (Didn't we previously wonder whether an expansion card could turn on the Lisa? Apparently it can, according to page 3-11 of the 1983 hardware manual.)

So what happens if you momentarily pull down RESET/ through a 1 kohm resistor? Well, the screen gets very bright and a bit smeared, but otherwise the Lisa runs normally. Huh. Resetting it with the real reset button brings the computer back to normal.

As a final amendment to the design for Mark 2, I'll probably just get rid of my big fills, which are not there for any particular reason and (as private feedback pointed out to me) may mostly just act as antennas.
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rayarachelian

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Re: The Great Extendo
« Reply #17 on: September 14, 2020, 11:59:15 am »

(Didn't we previously wonder whether an expansion card could turn on the Lisa? Apparently it can, according to page 3-11 of the 1983 hardware manual.)

Well done, and great find about the auto power on!
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patrick

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Re: The Great Extendo
« Reply #18 on: September 14, 2020, 05:03:37 pm »

Instead, it appears that (a) RESET/ can't be used that way, it's mostly an input that tells an expansion card to reset itself, and (b) if I wanted to wire RESET/ somewhere, I ought to have wired it to pull RESET/ up to +5VStdby, which can turn on the computer! (Didn't we previously wonder whether an expansion card could turn on the Lisa? Apparently it can, according to page 3-11 of the 1983 hardware manual.)

Oh, that's a clever trick. I've always wondered why they connected transistor Q11 the wrong way. A NPN with emitter towards VDD, that will not work. But during standby there is no VDD, so that rail will be at 0 V. In this case any high level applied to the base of Q11 will turn on the tranistor and connect /PWRSW (which is pulled up to the +5B2 standby voltage by R43) to ground. Same effect as pressing the power button on the front panel.


You cannot pull down the /RESET line to reset the computer. This would reset the I/O board and extension cards only, but not the CPU. There is buffer U2E on the CPU board, decoupling the /RESET line from the reset signal generated by U15B at startup or when the button has been pressed.


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