A travel through time for those interested in the history of the Disk ][ ...
One of my long-time goals was to try and replicate the Disk ][ controller that Woz originally designed.
Part of it was the challenge, part was to see what Woz went through and the differences that
this board has from the production board produced by Apple.
It took me a while to sort out those differences. You'll note there are a few less components
on the Woz built board as with Apple's production board. That, and the chips he used were
slightly different as well. I noticed on the oldest of Disk ][ production boards, Apple used
a 74LS259 in place of the 9330 chip used on most of the Disk ][ driver boards out there today.
Interestingly, from what I could tell, the schematic diagram Apple released was almost pin for
pin identical to the hand-built Woz board. Missing was the transistor "switches" that powered
down the D-Flop storage register and P5 PROM. On the Woz board, they were always powered on.
Also, it appears there is a missing pull-up resistor between pins 9 & 10 on the hand-wired board.
I could not see it in any archived photographs. The 74LS05 is an open-collector device and
would have likely contributed to some level of instability without that pull-up.
The two 555 ICs were replaced with a single 556, reducing the chip count.
A note-worthy story regarding the Apple ][ Disk Drive here:
You'll note that the article talks about Woz spending December 25th, 1977 working on the Disk ][
prototype, but if you look at the photo of the original board Woz built, it's dated 24-Feb-78.
Something is not right about the story. This distinction is also noted on Wikipedia:
https://en.wikipedia.org/wiki/Disk_II#Disk_II Everything I have found in searches indicates Woz
spending his Christmas Holiday building the Disk ][ interface, so why is the original board dated
24-Feb-78? Either this is not the original board, or he built this prototype as a pre-production
Conflict in facts ...
Here is a link talking about the original hand-wired (by Woz) Disk ][ board...
I like this article because it describes the board in-brief fairly well, though without photos.
I believe it was the description of the board when it went for sale several years ago for about 15K.
I believe Woz originally used this board to "settle" on his 13 sector scheme, as there is evidence on
the board itself that he moved some traces around using a different color wire wrap (blue noted) ...
likely because that's what was readily available to him at the time. The P5 PROM is missing from the board
which makes sense because that's the PROM that determines 13 sector from 16 sector R/W's to the disk.
Interesting article with a photo of the Disk ][ Drive & hand-built by Woz card:
PDF file showing the Disk ][ interface at a different angle. (zoom-able)
The only place I could find a decent photo of the back side of the board. (scroll down the page)
By itself: PCB back-side: https://bitrebels.com/wp-content/uploads/2013/02/apple-computer-rare-products-7.jpg
This was an interesting build. Worth my time. I enjoyed learning a bit of "hands-on" history.
The original Woz-built Disk ][ Driver board works fine on both 13 and 16 Sector, and I was able to
boot up ProDos without issue with the 16 sector PROM installed.
The 555 timer circuit pulse for the power-on Reset circuit was considerably shorter than the
production unit. This became obvious with the circuit using a 0.1uf capacitor on the Woz board vs.
a 22uf capacitor on the production board. Sometimes the computer wouldn't reset properly upon power-up.
The 74LS259 "8-bit latch" used in the Woz board is virtually identical to the 9334 chip used on the
production board. I'm unsure why they changed it but I'm guessing it had to do with timing consistency
or drive current. (fan-out)
The unknown: Why does the hand-built Woz board have a date of 24-Feb-78 when the story says Dec 25, 1977.
Comments welcome! Hope you enjoyed the write-up.
Picts: Original Woz hand-built board, followed by my hand-built replica.