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Based on a previous question about how slot 7 on the II / II+ was unique as it had two video signals on it that other slots lacked, I am curious why the slot wasn't taken more advantage of. But that aside, I used a few 80-column cards for the earlier machines and seem to recall that they were installed in slot 3 instead of 7.

It makes sense that slot 3 would be used given that it became the standard for the IIe, IIc and IIgs that followed, but the cards were designed before those machines came out and for the Apples that were from the late 1970s. But that being said, why would they have resided in that slot instead of #7 which had video signals available?

As a side note, I seem to recall that the Videx (?) card wouldn't clear the screen with a 'HOME' command and instead you'd have to pass the Control+L character instead. Then again, my mind is fuzzy 30+ years later :-)

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The signals available at slot 7 wouldn't be of any use for an 80-column card. On the one hand, a TV didn't have enough resolution to properly display 80 columns, so the TV color burst reference signal couldn't be used. On the other hand, on the Apple II and II plus, an 80-column card needed to use its own video RAM (using the mainboard video RAM would have interfered with the hardwired video output/RAM refresh). This is also the reason why the HOME command didn't work: it clears the mainboard video RAM, not the video RAM on the 80-column card. So some other way was needed to clear the 80-column screen, and an ASCII form feed character (Control-L) was an obvious choice, since the COUT (character out) routine pointed to the 80-column firmware ROM, anyway.

I'm not sure how slot 3 became the "standard" for 80-column card. Maybe the parallel printer card (slot 1) and the serial interface card (slot 2) where brought to market in that order, so the next free slot was slot 3. But I really don't know.

This "standard" was definitely established before the Apple //e, and when software like UCSD Pascal and CP/M started to use it, the //e had to take that into account.

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  • Would it theoretically have been of any advantage to someone trying to construct a "cheap" 80-column card to use the slot 7 signals to output an 80-column B&W TV signal? A monochrome output sent to a black-and-white TV generally has passable clarity for 80-column text. And if its theoretically possible, was it ever implemented by a third-party?
    – mnem
    Aug 4, 2016 at 20:17
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    @mnem: The color burst reference signal doesn't help if you want to output a B&W signal. The sync signal doesn't help if you don't want to use the original video signal (which just has 40 columns), it's easier to generate your own sync signals. So there's no way you can use these signals to make an 80-column card cheaper.
    – dirkt
    Aug 5, 2016 at 5:50
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    @mnem: I don't know if one needs the special slot signals or could drive them, but I would think one could construct a "cheap" 80-column card by piggy-backing on data fetches for hi-res graphics. The simplest approach, if one didn't mind having the first scan line of each text row blank, would be to use four 4517 chips (2x64 shift registers) to delay fetched data by a scan line, and then for scan lines 1,3,5,7 of character row output in each 40-column character cell the character chosen by the line-before data and the current-line data; on rows 2,4,6 show the characters in the other order.
    – supercat
    Nov 26, 2016 at 15:43
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    @mnem: Writing the upper-left character of the display would require storing its character code to $2000, $2800, $3000, and $3800; the next character would be $2400, $2C00, $3400, $3C00, then $2001, $2801, $3001, $3801, etc, but that would be a lot cheaper than trying to draw a 70-column graphics screen.
    – supercat
    Nov 26, 2016 at 15:51

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