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Years ago, I saw a book on how to put together a homebrew Motorola 68000 computer. One of the specific things the book talked about was grounding DTACK*, to make a very messy asynchronous bus design into a fully-synchronous, very simple design.

Try as I might, I can't find any reference to that book.

Does anyone recognize it?

senshin
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John R. Strohm
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    "DTACK Grounded" was the name of a hobbyist newsletter about the 68000, published in the early 1980's. – tcrosley Mar 06 '14 at 20:05

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I think tcrosley is on the right track with it being a hobbyist newsletter but the same small company that published it also sold 68000 co-processor boards using the DTACK name. There is a site here that has copies of some of the newsletters and the May 1985 edition contains the following price list:

YE DTACK GROUNDED PRICE LIST

DTACK Grande: 12.5MHz 68000: (for Apple II, 1 wait state)

less aux. bd: with aux. bd:

128K $650 640K $925

256K $690 768K $965

384K $730 896K $1005

512K $770 1024K $1045

Interestingly despite the name it mentions having one wait state so I'm not sure they ever achieved the DTACK grounded aim using memory technology of the day. I remember seeing their advertisements in a couple of magazines so that's probably where you recall the name from. I don't recall a design being released and the following is near the bottom of the price list:

(Like HP, DEC and Wang we consider our CPU circuitry proprietary)

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PeterJ
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  • The DTACK Grande was a later board that implemented a single wait state to allow the use of larger amounts of more affordable DRAM (up to 2MB, iirc). There was, in fact, an earlier DTACK Grounded product that used more expensive SRAM and did run with zero wait states. – mnem Jan 02 '20 at 03:27
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The book 'The Art of Electronics' (second editions) has a few chapters on building a 68008 based-computer; it mentions permanently grounding DTACK* on page 755.

jeroen74
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Are you thinking of the 68000 Hardware Manual by Peter A. Stark, which is the successor to his Radio-Electronics article series about building a 68000 single-board computer that fits the PC-XT form factor?

Among other things, one of the first steps in wiring up the CPU itself is to tie DTACK* low and ground the data bus. This will treat all of the address space as 0x0000, which corresponds to the instruction OR.B #$0,D0.

This causes the CPU to iterate over its entire address space as fast as it can and then wrap. The student can use a logic probe to see the address lines going high, each half as fast as the one next to it, as a cheap test that things are wired up correctly (to a basic degree).

(I've been re-reading the book, which is why I remember the details...)

Chris Hanson
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