What makes slot 7 of the Apple ][ unique from 1-6 and how was this used?

Question

In the Apple II, there are three unique slots (depending on model):

1. Slot 0 (on the ][ / ][+) which is typically used for language cards.
2. "Aux" slots in the //e and IIgs, typically used for memory expansion.
3. Slot 7 on all of them.

Slot 0 was primarily used for language cards and its uniqueness (from what I understand) is because the addressable space of that card overlaps with many motherboard functions (e.g. slots tend to get $Cx00-$CxFF for I/O, but things such as the toggle are mapped to $C030).

"Aux" slots probably deserve a separate question.

Leaving us with Slot 7, besides it being the first scanned during a cold boot, I believe I've read that it has signals that don't appear on any of the other slots such as a video line. If that is the case, I can't recall ever seeing any slot 7 specific cards.

Is slot 7 unique electrically? If so, is there anything that has ever taken advantage of this?

Answer 1

When the Apple ][ launched, it was marketed first in the USA. It was one of the first microcomputers to have built-in video circuitry for direct connection to a television monitor.

The television encoding was, as you might expect, the US NTSC standard (Incidentally, NTSC was mocked in Europe where it was said to stand for Never Twice the Same Colour). This meant that the machine could not connect directly to a European television.

Before Apple officially launched in the UK, ITT marketed a licensed copy - the ITT2020. This had a separate PAL (Phase Alternate Line) encoder card in slot 7. This card provided the UHF output for the television. Unfortunately this also had the side effect of altering the memory configuration and preventing some software running. PAL is 625 lines while NTSC is 525 lines, so extra memory was required for the graphics.

Apple shortly afterwards launched the Apple ][ model in Europe, including the UK. This official version had a different PAL encoder in slot 7 which altered the memory layout less, allowing more software to run correctly. I understand that a SECAM encoder was similarly provided for the French market, although I never saw one of these.

For example, I recall dumping my ITT2020 and getting an Apple ][ so that I could run VisiCalc.

Slot 7 was reserved for video cards. As @dirkt has already answered, it was unique in providing the video SYNC (pin 19) and COLORREF (pin 35) signals. SYNC was vital to provide the flyback pulse of the TV signal. If these signals had been daisy chained onto other slots, the degradation in their waveform would have resulted in a poorer quality picture. Any weakening of sync would have introduced jitter, or worse, into the viewed image. Also, at the time, the thought of wanting to drive anything other than one single TV was not considered viable so no effort was spent maintaining these signals onto other cards.

In time, third party suppliers did develop higher resolution video cards for slot 7.

Answer 2

Yes, slot 7 is unique electrically. Pin 19 (SYNC) and pin 35 (COLOR REF) are only connected for slot 7. These signals where needed by the PAL encoder card, which is why this card only worked in slot 7. I don't know of any other card that used these signals.

Edit: You asked in another question why the signals in slot 7 weren't taken advantage of by other cards. There's not really a lot you can do with them, except cater for some different TV encoding system. The color burst reference signal only applies to the TV color encoding, and the (horizontal) sync signal by itself is not enough to do anything interesting with it, you'd also at least need the vertical sync signal, or better, access to the horizontal and vertical counters, to do anything interesting video related.

Edit: I found another card that had to be installed in Slot 7, the Synetix Super Sprite Board, which can overlay the video output with sprites like other homecomputers could.

Answer 3

Light pens (Gibsen light pens for example) also need video sync so used slot 7.