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The Apple ][ Circuit Description:  A Review.........Bill Morgan

"Have you ever wanted to know the detailed circuit operation of your Apple ][ computer?  Perhaps you were designing a peripheral or making a modification.  Maybe you were repairing an Apple.  You may have just been curious about how it works."

That's the first paragraph of a new book called The Apple ][ Circuit Description, by Winston D. Gayler.  If the answer to that question is "yes", you need to look at this book.  Circuit Description contains about 160 pages of text describing the operation of every component on the Apple's motherboard and keyboard.  There are also 44 large fold-out pages of easy-to- read block diagrams, schematics, timing diagrams, and waveform drawings.  The enlarged, readable schematics alone will be worth the price of the book to some users!

One of the first things Mr. Gayler handles is identifying the various revisions of the Apple ][, from the original Rev. 0 through last year's RFI treated motherboard, Rev. D.  The body of the book covers that last version, while an appendix goes into the differences in all earlier revisions, and the diagrams show all revisions.  The very latest thing, the Apple //e, is not mentioned, since that's a radical departure from all others.

The book is intended for engineers, technicians, students, and serious hobbyists.  The descriptions, schematics, timing diagrams, and waveform drawings can be an invaluable help in designing peripherals and modifications, troubleshooting, studying practical circuit design, and just understanding how your Apple works.

Each chapter has two sections, Overview, and Detailed Circuit Description.  You can cruise the Overview sections to get an idea of what's going on in each piece of your Apple, or you can sit down with the Detailed Circuit Description, the schematics, your Apple, and your TTL Data Book, and figure out each and every signal in the computer.

Here is a chapter-by-chapter summary:

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1. Introduction and overview of the book.

2. Block-diagram discussion of the whole computer's structure, introducing concepts like "address multiplexer" and "video address generator".  Apple's unique patented power supply is also covered here.

3. Clocks: the master oscillator, clock generator, and the horizontal portion of the video address generator.  Clocks are especially important in the Apple due to their interplay with the video circuitry.

4. The vertical portion of the video address generator and the sync, blanking, and color burst signals.

5. RAM memory, the 4116's and their addressing, as well as the shared access scheme for the video memory.
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6. The 6502 processor and its internal cycles, including read cycles, write cycles, RAM and ROM cycles, I/O and keyboard cycles, interrupts, and DMA (direct memory access).

7. On-board I/O devices, including cassette I/O, the game port, the speaker, and the current two-piece keyboard.

8. Video generator hardware, how it creates TEXT, LORES, and HIRES displays under software control.

Appendices:

A. Introduction to standard video signal techniques, for those of us who know even less about video than about digital.

B. Various revisions of the Apple motherboard.  The main text of the book describes the RFI, Rev. D board.  This appendix covers the differences in all earlier boards, as well as the old one-piece keyboard.

C. Schematics.  Pages and pages of enlarged diagrams of all versions of the motherboard and keyboards.
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In the Introduction, Gayler says that the reader should be familiar with TTL (gates, flip-flops, shift registers, and multiplexers) and should have a basic knowledge of micro- processor and microcomputer architecture.  Well, I have a very basic knowledge of architectures, and almost no familiarity with TTL details.  This book looks like it will be a great tool for learning about TTL, because I will be able to relate what the data books say about a chip to a knowledge of what that chip is doing in my very own Apple.

One thing I would like to see is a sort of cross-reference by motherboard coordinate.  It would be nice to be able to ask the book "What is the function of that 74LS20 at location D2?"  As it is, I had to look through several foldouts for a chip symbol labelled "D2".  It is a NAND gate in "Fig. C-2. Clock Generator (all revisions)"  Since it's part of a clock circuit, it must be covered in chapter 3.  Several minutes of poking around in chapter 3 tells me that chip is part of one of the Apple's most unique features!  Every 65th CPU cycle is slightly stretched (1117 us vs. 978 us) to maintain sync with the color signals, and D2 is responsible for triggering that stretch.

That last paragraph started out to describe a shortcoming of the book, and turned into yet another example of the kind of great information contained in The Apple ][ Circuit Description.  If you're doing any hardware work with the Apple, or if you want to learn more about what's going on in there, you need this book.

The Apple ][ Circuit Description, by Winston D. Gayler.  Published by Howard W. Sams.  8 1/2 by 11 comb binding.  172 pp. text, 44 fold-out diagrams.  Shipping weight 3 lbs.  List price is $22.95, our price will be $21 + shipping ($2 domestic, $12 overseas).
