Notes re: AT+T UNIX PC / 3B1 Reference Manual and Service Manual

Once upon a millennium past, AT&T tried to put protected virtual memory and preemptive multiprocessing on desktops. The world was not yet ready.

AT&T UNIX PC a.k.a. PC 7300 a.k.a. 3B1 a.k.a. (Convergent) S/50 a.k.a. S4 a.k.a. Safari 4

This is mostly about the AT&T UNIX PC Reference Manual and AT&T UNIX PC Service Manual. It's all the result of one guy's brief fit of "research" and shouldn't be trusted. For brevity I'm just going to say stuff like it's true. Caveat lector.

Reference Manual

There are a few scans of the Reference Manual online.  The most useful is https://web.archive.org/web/20260928201015/https://6reoquestions.s3.us-east-2.amazonaws.com/misc/UnixPC/UNIXPC_Ref_ocred.pdf. [2]

• OCR processed -- searchable
• complete -- i.e. all the same content as my real copy
• mostly in order -- i.e. mostly same order as my real copy except:
  • appendices to the Reference Manual proper are at the end after all the extra stuff
  • schematics in different order -- but order appears arbitrary so no matter
• the scan is good
  • text is fine
  • schematics are sufficiently legible for sane perusal
  • insane fixation on minutia might warrant a real copy or better scan
• might be scanned from early copy
  • title page is simple text instead of dressy color graphic card stock
  • no evidence any updates to content regardless of date or packaging
• 488 pages (for quick differentiation from other scans)
• content by PDF page:
  

  Reference Manual:
  4	preface / ToC
  14	Ch 1: System Features and Functions
  38	Ch 2: Logic Board Theory of Operation
  144	Ch 3: Diagnostics
  174	Ch 4: Logic Board Test Procedures
  Reference Manual: schematics
  228	video i.c.
  232	DMA address i.c.
  234	D-08-00222-00: "S4, CPU" [0.5MB as four banks of 64k-word with 256k/2MB option described]
  262	D-08-00225-00: "S4, CPU P5 (1.0MB)" [as two banks of 256k-word)
  288	D-08-00230-00: "S4, CPU" [um... see note 3]
  Reference Manual: appendices at end
  additional topics:
  314	Floppy Tape Interface Theory of Operation
  336	RAM-COMM Expansion Board
  346	International Version Reference Manual Supplement
  352	DOS-73 Technical Reference
  382	Ethernet Expansion Board Theory of Operation
  418	S4BUS - AT&T UNIX PC EXPANSION BUS SPECIFICATIONS
  Reference Manual: appendices
  458	A - PAL Equations
  472	B - Mnemonics
  480	C - Expansion Memory Locations

• for search:
  • I don't know what AT&T calls this designator: 999-300-189IS [4]
  • or this one: PE 1620-127 REF MAN
  • Part number / "comcode": 845660117
• for credit:
  • original scan http://www.unixpc.org/UNIXPC_Ref.pdf made 2/22/2006 by corey@lod.com
  • OCR version generated and published by "Forgotten Machines" historian AJ at https://unixpc.blogspot.com/ -- AJ's contributions will come up again on this page

About the Schematics...

After reading way too much of this [see notes generally but do you really want to go there?], I have some questions and a conjecture.

The D-08-00225-00 drawings for "S4, CPU P5 (1.0MB)" appear to be the most "evolved" or latest state of the main board while the "D-08-00222-00" drawings -- one of two sets titled "S4, CPU" -- reflect something closer to early production hardware.

I wonder if (Convergent for) AT&T produced any boards that could not have been configured for and stuffed with any of the 0.5MB/1MB/2MB RAM options. Maybe early boards allowed only 0.5MB or 2MB not 1MB. If so, I wonder if any later boards that added support for 1MB also removed support for 0.5MB. If any did, then I wonder if all did.

The May 1985 Byte article includes:

Main Memory
512K bytes of dynamic RAM with parity bit on motherboard,
currently expandable to 2 megabytes via expansion boards;
machine's design allows for a maximum of 4 megabytes
...
Optional Hardware...

Notes distracting from AT+T Reference Manual

If you're scrolling...

...or scroll ahead a chunk

because that was the point of this page already up there.

Probably. Because if you're scrolling, you probably scrolled to this under that.

This is just notes for that. On a separate "Page" because this rabbit hole went deeper that HaD's max Page length.

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Here would be a fine place to stop reading.

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Beware deeper rabbit holes ahead.

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Mythos limits sanity.

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You can still turn back.

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Notes

note 1 (photo source)

Those photos are widely used. The earliest instance I found is this 2011 snapshot from Steve's Old Computer Museum. I'm inclined to bet on Steve as the source because a) a couple of old-ish uses that found in a quick image search that cite a source cite oldcomputers.net, and b) the 2011-vintage images carry the unmasked original photos as embedded thumbnails (low res).

note 2 (ref manual scans)

• http://www.unixpc.org/UNIXPC_Ref.pdf
  • best scan
  • scanned and hosted by corey@lod.com
  • check out corey's site unixpc.org!
  • 488 pages (for quick id of different scans)
• https://web.archive.org/web/20260928201015/https://6reoquestions.s3.us-east-2.amazonaws.com/misc/UnixPC/UNIXPC_Ref_ocred.pdf
  • OCR'ed version of scan by corey@lod.com / unixpc.org above
  • OCR by AJ / Forgotten Machines
    • "I think I used Linux's OCRMYPDF utility to do that" (email Oct 2026)
    • found by link from one of AJ's "Forgotten Machines" blogs https://unixpc.blogspot.com/p/technical-reference-manual.html
  • favorite example -- see main text
  • also 488 pages
• https://bitsavers.org/pdf/att/3b1/hardware/999-300-1891S_UNIX_PC_Reference_Manual_1986.pdf
  • the large collection of high quality scans at bitsavers.org appears to be largely the work of one person: Al Kossow who is also the Robert N. Miner Software Curator at the Computer History Museum [2.1]
  • without diminishing Mr. Kossow's work, this version isn't the best match to the real manual
    • additional topics included with real copy and unixpc.org scan are not included, except:
      • International Version Reference Manual Supplement, which is inserted between appendices and schematics
      • separately in the same directory:
        • DOS-73 Technical Reference
        • Ethernet Expansion Board Theory of Operation
        • RAM-COMM Expansion Board
      • which leaves missing:
        • Floppy Tape Interface Theory of Operation
        • S4BUS - AT&T UNIX PC EXPANSION BUS SPECIFICATIONS
    • schematics follow appendices (& int'l version ref) - present just different from ToC
    • schematic order differs from my real copy and unixpc.org scan (not a big deal)
    • "I" -> "1"  error in filename
  • scan quality is similar to unixpc.org scan
    • text is fine
    • looking for legibility threshold in schematics: some details more legible in unixpc.org scan and others more legible in this scan
  • also: https://archive.org/details/bitsavers_att3b1hardPCReferenceManual1986_13267438 in collection https://archive.org/details/bitsavers_att
  • 341 pages (for quick id of different scans)
• https://bitsavers.org/pdf/att/3b1/reference_manual/
  • also at bitsavers.org
  • pro: appears to include all content of real manual
  • con: lower quality scan
  • like or leave: directory of multiple files
    • harder to associate with order of content in real manual - which maybe doesn't matter
    • easier to pick out topics other than unnamed chapters of the Reference Manual proper
  • added notations to "cpu" schematic filenames appear confused
    • "ps" in "3b1_s4_cpu_ps_(1.0MB).pdf" looks like mistranscription of "p5"
      • if true, then:
    • "_ps_*" additions to other two "cpu" schematics are incorrect for non-P5 boards
    • "_(2.0MB)" addition to "3b1_s4_cpu_ps_(2.0MB)" is incorrect for 0.5MB board (with notes for conversion to 2MB)
• https://web.archive.org/web/20240607211248/https://6reoquestions.com/misc/S50/S50_Reference_Manual.pdf
  • an interesting curiosity: AT&T UNIX PC Reference Manual in a Convergent Technologies...

link targets

posted here so I can hit them from elsewhere

uncaptioned "and there was much rejoicing" GIFs

484 x 290 -- 1.33 MB

542 x 324 -- 1.59 MB

638 x 382 -- 2.08 MB

638 x 382 -- 2.5 MB