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When I started with the Mac team in Jan '81 there was just one wire-wrapped 68000 prototype, so my first job was to start making more protos along with Brian ('Uncle Dougie') Howard - who technically was a writer working under Jef but had been handling many of the hardware build-and-debug tasks for Burrell. Between the two of us Brian and I built another 4 or 5 prototypes for use by the programmers. These were built using some generic 8" x 9" printed circuit cards we found. One of these wire-wrap protos could be built in a couple of days if you had all the parts on hand, and they had the advantage of being readily modifiable. Actually, these protos were undergoing fairly continual modification and the one I that I held on to, #5, had been updated in 1982 with the Z8530 SCC (Serial Comm Controller), the IWM (Integrated Woz Machine) disk controller, and the STF (Special Task Force) clock, which were improvements over the original design.
The basic Mac architecture was very spare, (**"TIGHT!"** as the teenagers now say): about 32 ICs not counting the 68000 and the 16 RAM chips. There was the Timing State Machine (TSM), made out of a PAL and some flipflops; the Linear Address Generator (LAG) made out of a PAL and a couple of counters; the Bus Mgmt Unit (BMU) PAL, 4 multiplexors for the RAM addressing, 2 EPROMs, some bus drivers, a video output shift register, and the 6522 Peripheral Interface Adapter (PIA) which had 16 programmable I/O lines, which handled the keyboard and mouse interfaces and some other tasks. That was the core architecture, to which was added the dual serial ports, the internal and external floppy drive ports, the real-time clock chip, and the sound output. The PALs were the Programmable Array Logic parts made by MMI, which allowed one to write output logic equations to define each of the 8 outputs. They were fairly power-hungry (by today's standards at least) but were cheap and flexible and Burrell put them to good use.
The underside of the wire-wrap protos was of course a maze of wires. One morning we were amazed to find that one of the protos was badly scorched... it turned out that late the night before, Burrell had been making some changes and managed to get power and ground shorted together somehow, and in his impatience to get the proto back up, he decided to use 'brute force' by removing all the chips and connecting 120VAC across the power and ground in an attempt to 'burn out' the short.... a misguided attempt, as one would expect! I don't that proto was ever revived.
Later in 1981 we brought Colette Askeland over from the Apple-II/III division to lay out a Mac PCB - the first of eight iterations of the board between 1981 and the final version in late 1983. Looking at these boards now, they seem like quaint antiques indeed. Not one surface mount component! Here's a list of the 8 board iterations:
--Close to the final version, this board now has 5 PALs: the TSM, LAG, BMU0 and BMU1, and the ASG (Analog Sound Generator). It also has a 4-pin connector (J8) that I recall we added to enable bringing out a few key video signals to allow for creating color or grey-scale video in the future.
Diagram of Macintosh circuit board schematic
The Macintosh schematics were drawn by Dave Root (I think) in the CAD group, under the direction of lead mechanical designer Jerry Manock. Following industry practice, the set of official Mac schematics comprised something like 3 or 4 large 'D' size drawings, with many signals crossing from one page to the other and back again. As the Mac electrical design became frozen in early 1983 I was thinking we needed a simplified condensed schematic, a one-page version that could be reduced and hung on the wall when troubleshooting the logic board. Since so much of the Mac architecture was incorporated in PALs and every input and output of a PAL has a name, it occurred to me to make a new kind of hybrid schematic, where the signal paths were not actually drawn (except in a few cases), but there's a name for every input and output of every chip on the board. This would be a hybrid of the assembly drawing, which shows every part in its actual location and orientation on the board, and the schematic, which shows what connects to what.
The problem with traditional schematics is that what you usually want to know is where to find a particular signal to look at it, so what you really need is an easy way to find the pin number and where on the chip it is - something the traditional schematics are not that helpful for since the pins on each chip are usually drawn in a function-oriented (or else random) way, rather than drawing the pinout as it actually appears when you look at the part. Imagine a stylized assembly drawing, where each pin of each part has a named signal - that's the schematic I made. Along with a netlist which listed what each pin connects to, of course - but usually you'd pretty much have that memorized if you were familiar with the architecture.
The Mac CAD Dept didn't have any use for this kind of hybrid document, and asked me to put a note on it to the effect that it wasn't a released document but a technical aid only. That was fine with me, I just found it really satisfying to be able to work on diagnosing board problems with just that one piece of 11 x 17 paper (reduced from one 'D' page) instead of fumbling around with multiple D-size drawings. Nowadays schematics ARE a lot more like that, except still they're usually spread over many 8.5 x 11" pages and still don't give any help w/r to finding pins on large surface-mount chips.
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