Image Map Image Map
Results 1 to 10 of 10

Thread: DEC register 8271

  1. #1
    Join Date
    Dec 2013
    Location
    Near Milwaukee Wisconsin
    Posts
    868

    Default DEC register 8271

    The PDP8E uses the DEC 8271 for the AC, MB and other registers. For some reason, I can not find the datasheet for this IC. I thought I had it but can not find it. Anyone have it or a link to it? Thanks Mike.

  2. #2

    Default

    As is common it's an old Signetics part.
    See here http://www.datasheets360.com/part/de...3221418750331/ for instance.

    (Common failure too in my experience ;( ).

  3. #3
    Join Date
    Dec 2010
    Location
    Seattle, WA
    Posts
    1,545

  4. #4
    Join Date
    Dec 2013
    Location
    Near Milwaukee Wisconsin
    Posts
    868

    Default

    AAAhhhhh..... Thanks gentlemen. That's were I had a copy of the information. I have a paper copy of that Signetics book. With these fancy computers, their cavernous space and easy search, I forget about my paper stuff. Thanks again Mike

  5. #5

    Default

    Tangent: What's the reason for using a shift register for an accumulator and memory buffer register? I realize the shift function is a natural for an alu operation, but there are many other operations like add/sub/inv/inc/dec.
    Is there a tricky reason they used a the 8271 shift register?

  6. #6
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    1,520

    Default

    Pretty much the only thing that is required for the PDP-8 architecture are registers, an adder, a shifter and some selection gates (also effectively implementing an OR function).

    A nice and simple architecture...

    Dave

  7. #7
    Join Date
    Aug 2010
    Location
    Silicon Valley USA
    Posts
    633
    Blog Entries
    4

    Default

    Quote Originally Posted by Ragnarock View Post
    Tangent: What's the reason for using a shift register for an accumulator and memory buffer register? I realize the shift function is a natural for an alu operation, but there are many other operations like add/sub/inv/inc/dec.
    Is there a tricky reason they used a the 8271 shift register?
    Probably the whim or experience of the designer that did that board. On the timing board they used the 74194 which could functionally replace the 8271. The control input encoding is slightly different for the 74194 vs the 8271, but not really significant. The 74194 can do both left/right shift, whereas the 8271 only does right shift.

    It might have been a cost thing (ie, the 8271 could have been a few bucks cheaper back then, who knows).

    However from today's perspective you can still buy 74xxx parts off the shelf. 82xx parts are basically nonexistent. So in hindsight a 74194 might have been a better choice.

    But I don't think the DEC folks were thinking what about 40 years out for spare parts for these boards ...

    Don

  8. #8

    Default

    74179 is the 74 series equivalent of the N8271.

  9. #9
    Join Date
    Dec 2013
    Location
    Near Milwaukee Wisconsin
    Posts
    868

    Default

    So..... I still have a couple of 8271 spares left. I could use the 74179 as a replacement when I run out? Mike

  10. #10

    Default

    I'm using a 74HC193 counter for the PC, SP and index registers in a cpu design. Seems reasonable.
    4-bit, up/dn, parallel load.
    I can do AND/OR/INV/SHIFT with other logic, but this handles INC/DEC.
    In fact, shift is just a loading of all the bits miswired, same with nibble swap, so no real logic there, just connections.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •