Macintosh Bus Master Nubus Card 820-0199-C, 077-8264

[zigzagjoe]

(not my picture)

Took a moment to dig into this earlier today after seeing one of these on ebay. This card may be found under the P/Ns 820-0199-C, 077-8264, probably others. It is a component of the AppleCAT test kit. Specifically, it is used to test that Nubus bus mastering works correctly.

The card itself appears to be extremely simple, with some registers to store a 32 bit address and data along with PALs to work the nubus. The bus master card must be installed in slot $A (Second nubus slot) per the documentation (found here). The DeclROM contains no code, just a basic ROM to announce "I am here" to slot manager and nothing else (as expected).

Theory of operation: AppleCAT is an early version of the Techstep, it sends payloads over serial (Found as KODE resources in the AppleCAT binary) to the system under test which executes them and indicates success/fail. The magic bit of code here is sub_6BC; it writes an longword address to a register of the card in Slot $A, along longword of data, then sends a trigger to a third address. Looks to me like that trigger can have bit 8 set to cause a write, otherwise it performs a read. The routine waits a bit (nops) during which the card i assumed to be doing its thing, then checks a provided address against an expected value to verify the requested operation was performed using bus mastering. The routine loc_8D8 sets up both a read and a write test using that routine.

Long story short, all the bus master card is used for is to (upon a deliberate trigger) perform a data read from an address into its register, or to write data from its register to that address. It otherwise does nothing but look cool. TLDR: It's a diagnostic card, nothing more, doesn't enable any interesting functionality nor does it accelerate the machine in any way.



Attached is a quick IDA disassembly, a copy of the ROM, a fixed floppy image, and some other bits relevant.

Sources:

https://macintoshgarden.org/apps/applecat-ii

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Hello everyone I don't know if most of you have heard of the Apple C.A.T. Diagnostic Tool, it was issued form Apple in the mid to late 80's and was used to diagnose and fix computers such as the Macintosh Portable, 128k, 512k(e), Plus, SE, SE/30, ii, iix, iicx, and iici (there may be more I am...

tinkerdifferent.com

Apple CAT Documentation/Guides : Apple Computer : Free Download, Borrow, and Streaming : Internet Archive

All of the documentation for the AppleCAT Diagnostic Tool

archive.org

 

[cheesestraws]

Oh, interesting. I wonder where I got that idea from, then. I stand corrected! Thankyou.

 

[olePigeon]

Thanks for this info. I had read many different explanations as to what it was used for, including "fixing" or "enabling" Bus Mastering on certain Mac II models to speed them up.

 

[RepairManiac]

Thank you so much for sharing!!!!

I can't for the life of me remember this board or this particular diagnostic. So, I will need to visit my former boss to see if we ever had AppleCAT or other items such as TechStep Diagnostics. At that time, we were involved in Pilot programs, such as helping to develop better diagnostics and testing new programs before they were released to dealers across the country. I do remember sending certain failed criteria logic board to be reviewed by a special group that were working on making diagnostics more robust. They would report back and sometimes they couldn't duplicate the problem even though a board replacement fixed the issue.