Re: lisa 2 startup problems

>How is the video signal/circuit influenced by the IO board? even if
>the cpu boards starts and gives the error 41, the image on the
>monitor is still not right, the while lines are present and the top
>while bar is not in a "very good" shape, this was the first symptom
>and I thought there must be something wrong with the data or address
>bus, but now the data and address bus lines on the IO Board are
>"disconnected" from the rest of the components.

The Lisa design includes software control of the brightness of the screen (this is referred to as "contrast" in the Lisa documents but most people would consider it "brightness").

The signal that controls the brightness comes from a D-A converter circuit on the I/O board (U9B & RP3 & U10A). Without a functional I/O Board, the brightness is likely to go to the extreme, and even after adjusting the controls on the Power Supply, the screen may look fuzzy and/or display the diagonal retrace lines.

The "contrast latch" (U9B) data is written via the parallel port VIA (U5C), so if you remove this VIA from an otherwise functional I/O Board, the video will be affected.

You could supply your own brightness level by injecting a DC voltage in the range of 1 to 5V at J1-87 with the I/O Board removed.

>I can also notice a "slow" refresh of the white box with the cpu
>error at about 1 Hz,

I have not looked into this symptom in any detail, but without a functional I/O Board, the CPU cannot receive mouse and keyboard input, and it is possible the ROM code does a form of reset when it realizes this.

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