Why 2 x 8 bit unidirectional S-100 bus? - CP/M

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Thread: Why 2 x 8 bit unidirectional S-100 bus?

  1. Why 2 x 8 bit unidirectional S-100 bus?


    Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    buffers instead of bidirectional? Seems wasteful as every board
    I've seen only converts them back to bidir.

    I've been trying to figure out possible reasons for days now, and
    found no explanations in Bursky's or Garetz's books. My guess is
    there's either no reason, or something to do with front panel.

    ~ J


  2. Re: Why 2 x 8 bit unidirectional S-100 bus?

    MdntTrain wrote:
    > Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    > buffers instead of bidirectional? Seems wasteful as every board
    > I've seen only converts them back to bidir.


    My understanding is that MITS got a good deal on 100 pin connectors.

    My guess, then, is that they tried to find a use for those pins.
    Much of S-100 seems to be an accident.

    Note also that man y RAM chips at the time had separate data
    in and out pins.

    -- glen


  3. Re: Why 2 x 8 bit unidirectional S-100 bus?

    glen herrmannsfeldt wrote:
    > MdntTrain wrote:
    >> Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    >> buffers instead of bidirectional? Seems wasteful as every board
    >> I've seen only converts them back to bidir.

    >
    > My understanding is that MITS got a good deal on 100 pin connectors.
    >
    > My guess, then, is that they tried to find a use for those pins.
    > Much of S-100 seems to be an accident.
    >
    > Note also that man y RAM chips at the time had separate data
    > in and out pins.


    A couple of the minicomputer busses of the time also had separate data
    lines (DEC Unibus comes to mind).

    Bob

  4. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On 2007-08-30, BobH wrote:
    > glen herrmannsfeldt wrote:
    >> Note also that man y RAM chips at the time had separate data
    >> in and out pins.

    >
    > A couple of the minicomputer busses of the time also had separate data
    > lines (DEC Unibus comes to mind).


    UNIBUS did not have separate in and out data lines. Neither did
    OMNIBUS, for that matter, although it did have separate data busses for
    memory and peripherals.
    --
    roger ivie
    rivie@ridgenet.net

  5. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On Wed, 29 Aug 2007 13:08:52 -0700, MdntTrain wrote:

    >
    >Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    >buffers instead of bidirectional? Seems wasteful as every board
    >I've seen only converts them back to bidir.
    >
    >I've been trying to figure out possible reasons for days now, and
    >found no explanations in Bursky's or Garetz's books. My guess is
    >there's either no reason, or something to do with front panel.
    >
    >~ J


    Made it easier to do simple output or input devices also simplified
    the data hold time for read or write. The origin was likely the
    basic MDS8 and later the 8080 version that used a bus that was
    mostly CPU signals buffered minimally. It was a side effect of
    both CPUs having control signals and the like multiplexed on the
    CPU data bus. You need to buffer the CPU databus for output data
    as it has two paths, one is a latch to catch the CPU state and cotrol
    signals and the other is DATA out to the bus on writes.

    The front pannel logic was not really simplified any by it though it
    didn't hurt.

    The amount of waste is similar to bidirectional busses you have
    to buffer them and it takes about the same number of parts.

    The down side was there were designs that didn't buffer the input
    making for noise and capacitive loading problems (a bad thing).
    Usually it was early 2102 ram cards that did that but there were a few
    IO that weren't pretty.

    Mostly that was how people did it then. Later when the 8228/8238
    appeared and the few 8080 designs that used it tended to have
    bidirectional data busses... I suspect so intel could sell the 8216
    (4bit bidirectional buffer).

    Allison

  6. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On Aug 30, 7:29 am, no.s...@no.uce.bellatlantic.net wrote:
    > On Wed, 29 Aug 2007 13:08:52 -0700, MdntTrain wrote:
    >
    > >Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    > >buffers instead of bidirectional? Seems wasteful as every board
    > >I've seen only converts them back to bidir.

    >
    > >I've been trying to figure out possible reasons for days now, and
    > >found no explanations in Bursky's or Garetz's books. My guess is
    > >there's either no reason, or something to do with front panel.

    >
    > >~ J

    >
    > Made it easier to do simple output or input devices also simplified
    > the data hold time for read or write. The origin was likely the
    > basic MDS8 and later the 8080 version that used a bus that was
    > mostly CPU signals buffered minimally. It was a side effect of
    > both CPUs having control signals and the like multiplexed on the
    > CPU data bus. You need to buffer the CPU databus for output data
    > as it has two paths, one is a latch to catch the CPU state and cotrol
    > signals and the other is DATA out to the bus on writes.
    >
    > The front pannel logic was not really simplified any by it though it
    > didn't hurt.
    >
    > The amount of waste is similar to bidirectional busses you have
    > to buffer them and it takes about the same number of parts.
    >
    > The down side was there were designs that didn't buffer the input
    > making for noise and capacitive loading problems (a bad thing).
    > Usually it was early 2102 ram cards that did that but there were a few
    > IO that weren't pretty.
    >
    > Mostly that was how people did it then. Later when the8228/8238
    > appeared and the few 8080 designs that used it tended to have
    > bidirectional data busses... I suspect so intel could sell the 8216
    > (4bit bidirectional buffer).
    >
    > Allison


    I tend to agree with this assessment, and there were some cards like
    the Northstar MDS disk controller that really overloaded the use of
    the data in bus, by using "reads" to actually write to the hardware by
    using the lower 8 bits of the address as the write data. Of course
    this was st the expense of wasting precious space in the address map,
    but it did simplify the hardware, and cut costs. I guess this was a
    reasonable approach when 64k was a lot of memory.

    -Howard


  7. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On Thu, 30 Aug 2007 13:21:20 -0700, "hharte@hartetec.com"
    wrote:

    >On Aug 30, 7:29 am, no.s...@no.uce.bellatlantic.net wrote:
    >> On Wed, 29 Aug 2007 13:08:52 -0700, MdntTrain wrote:
    >>
    >> >Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    >> >buffers instead of bidirectional? Seems wasteful as every board
    >> >I've seen only converts them back to bidir.

    >>
    >> >I've been trying to figure out possible reasons for days now, and
    >> >found no explanations in Bursky's or Garetz's books. My guess is
    >> >there's either no reason, or something to do with front panel.

    >>
    >> >~ J

    >>
    >> Made it easier to do simple output or input devices also simplified
    >> the data hold time for read or write. The origin was likely the
    >> basic MDS8 and later the 8080 version that used a bus that was
    >> mostly CPU signals buffered minimally. It was a side effect of
    >> both CPUs having control signals and the like multiplexed on the
    >> CPU data bus. You need to buffer the CPU databus for output data
    >> as it has two paths, one is a latch to catch the CPU state and cotrol
    >> signals and the other is DATA out to the bus on writes.
    >>
    >> The front pannel logic was not really simplified any by it though it
    >> didn't hurt.
    >>
    >> The amount of waste is similar to bidirectional busses you have
    >> to buffer them and it takes about the same number of parts.
    >>
    >> The down side was there were designs that didn't buffer the input
    >> making for noise and capacitive loading problems (a bad thing).
    >> Usually it was early 2102 ram cards that did that but there were a few
    >> IO that weren't pretty.
    >>
    >> Mostly that was how people did it then. Later when the8228/8238
    >> appeared and the few 8080 designs that used it tended to have
    >> bidirectional data busses... I suspect so intel could sell the 8216
    >> (4bit bidirectional buffer).
    >>
    >> Allison

    >
    >I tend to agree with this assessment, and there were some cards like
    >the Northstar MDS disk controller that really overloaded the use of
    >the data in bus, by using "reads" to actually write to the hardware by
    >using the lower 8 bits of the address as the write data. Of course
    >this was st the expense of wasting precious space in the address map,
    >but it did simplify the hardware, and cut costs. I guess this was a
    >reasonable approach when 64k was a lot of memory.


    Yes, but it was properly buffered and interfaced to the bus.
    The fact that it chewed up E800h though EFFFh is only 2k and
    in the early years most people would have killed for even
    56k of ram. Also that 2k included the Boot roms. It was resonable
    when SA400s were still $399 each and a controller was another $500.

    FYI: it was possible to relocate the MDS to F800h ( I have a set of
    proms around that did that.). It was very easy to do. Also NS* for a
    price would deliever a custom prom set and matching DOS for any
    address combination.

    When I refered to data bus loading I was talking in DC and AC terms
    as in 74367s could only drive so much and 8 ram card amounting
    to 64 rams per data bit line (64k of 8kx8 2102s) if they were not
    buffered but a huge AC load on the bus. Some of the early ram cards
    were not so good or at least tolerable if you only had one or two.

    Allison
    >-Howard



  8. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On Aug 29, 5:27 pm, glen herrmannsfeldt wrote

    On Aug 29, 7:32 pm, BobH
    wrote

    On Aug 29, 8:56 pm, Roger Ivie wrote

    On Aug 30, 3:36 pm, no.s...@no.uce.bellatlantic.net wrote

    On Aug 30, 3:21 pm, "hha...@hartetec.com" wrote
    -------------------------------------
    Thank you all for your cool and informative replies!

    ~ J



  9. Re: Why 2 x 8 bit unidirectional S-100 bus?


    "MdntTrain" wrote in message
    news:1188418132.136470.243670@50g2000hsm.googlegro ups.com...
    >
    > Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    > buffers instead of bidirectional? Seems wasteful as every board
    > I've seen only converts them back to bidir.
    >
    > I've been trying to figure out possible reasons for days now, and
    > found no explanations in Bursky's or Garetz's books. My guess is
    > there's either no reason, or something to do with front panel.
    >
    > ~ J

    I may be stepping into a pile of something stinky, but it seems it was as
    logical "back in the day" as it is now to use the manufacturer's application
    notes and example architecture to implement new technologies, and I believe
    such was the case with almost all early implementers of the Intel
    microprocessors. I'll have to go back to my early data books to confirm,
    but I seem to recall that the Intel MCS system and variants laid the
    groundwork that was loosely followed by the esteemed and enshrined Mr.'s
    Titus and Roberts and numerous others who chose the 8080 platform to
    exploit.



  10. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On Thu, 13 Sep 2007 12:05:37 -0700, "Thomas \"Todd\" Fischer"
    wrote:

    >
    >"MdntTrain" wrote in message
    >news:1188418132.136470.243670@50g2000hsm.googlegro ups.com...
    >>
    >> Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    >> buffers instead of bidirectional? Seems wasteful as every board
    >> I've seen only converts them back to bidir.
    >>
    >> I've been trying to figure out possible reasons for days now, and
    >> found no explanations in Bursky's or Garetz's books. My guess is
    >> there's either no reason, or something to do with front panel.
    >>
    >> ~ J

    >I may be stepping into a pile of something stinky, but it seems it was as
    >logical "back in the day" as it is now to use the manufacturer's application
    >notes and example architecture to implement new technologies, and I believe
    >such was the case with almost all early implementers of the Intel
    >microprocessors. I'll have to go back to my early data books to confirm,
    >but I seem to recall that the Intel MCS system and variants laid the
    >groundwork that was loosely followed by the esteemed and enshrined Mr.'s
    >Titus and Roberts and numerous others who chose the 8080 platform to
    >exploit.
    >


    You are correct! One only has to look at the MCS80 manual to see
    that.

    Allison

  11. Re: Why 2 x 8 bit unidirectional S-100 bus?


    wrote in message
    news:96kje39dtqsvg8brbamqc5u34lj4n3r60c@4ax.com...
    > On Thu, 13 Sep 2007 12:05:37 -0700, "Thomas \"Todd\" Fischer"
    > wrote:
    >
    >>
    >>"MdntTrain" wrote in message
    >>news:1188418132.136470.243670@50g2000hsm.googlegro ups.com...
    >>>
    >>> Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    >>> buffers instead of bidirectional? Seems wasteful as every board
    >>> I've seen only converts them back to bidir.
    >>>
    >>> I've been trying to figure out possible reasons for days now, and
    >>> found no explanations in Bursky's or Garetz's books. My guess is
    >>> there's either no reason, or something to do with front panel.
    >>>
    >>> ~ J

    >>I may be stepping into a pile of something stinky, but it seems it was as
    >>logical "back in the day" as it is now to use the manufacturer's
    >>application
    >>notes and example architecture to implement new technologies, and I
    >>believe
    >>such was the case with almost all early implementers of the Intel
    >>microprocessors. I'll have to go back to my early data books to confirm,
    >>but I seem to recall that the Intel MCS system and variants laid the
    >>groundwork that was loosely followed by the esteemed and enshrined Mr.'s
    >>Titus and Roberts and numerous others who chose the 8080 platform to
    >>exploit.
    >>

    >
    > You are correct! One only has to look at the MCS80 manual to see
    > that.
    >
    > Allison


    Correct that I stepped into something stinky, or that adoption of new
    architecture generally followed preliminary guidelines?

    As always, Allison, your insight and citations are a treasure!



  12. Re: Why 2 x 8 bit unidirectional S-100 bus?

    On Thu, 13 Sep 2007 17:31:18 -0700, "Thomas \"Todd\" Fischer"
    wrote:

    >
    > wrote in message
    >news:96kje39dtqsvg8brbamqc5u34lj4n3r60c@4ax.com...
    >> On Thu, 13 Sep 2007 12:05:37 -0700, "Thomas \"Todd\" Fischer"
    >> wrote:
    >>
    >>>
    >>>"MdntTrain" wrote in message
    >>>news:1188418132.136470.243670@50g2000hsm.googlegro ups.com...
    >>>>
    >>>> Does anyone know why the S-100 bus used 2 x 8 bit unidirectional
    >>>> buffers instead of bidirectional? Seems wasteful as every board
    >>>> I've seen only converts them back to bidir.
    >>>>
    >>>> I've been trying to figure out possible reasons for days now, and
    >>>> found no explanations in Bursky's or Garetz's books. My guess is
    >>>> there's either no reason, or something to do with front panel.
    >>>>
    >>>> ~ J
    >>>I may be stepping into a pile of something stinky, but it seems it was as
    >>>logical "back in the day" as it is now to use the manufacturer's
    >>>application
    >>>notes and example architecture to implement new technologies, and I
    >>>believe
    >>>such was the case with almost all early implementers of the Intel
    >>>microprocessors. I'll have to go back to my early data books to confirm,
    >>>but I seem to recall that the Intel MCS system and variants laid the
    >>>groundwork that was loosely followed by the esteemed and enshrined Mr.'s
    >>>Titus and Roberts and numerous others who chose the 8080 platform to
    >>>exploit.
    >>>

    >>
    >> You are correct! One only has to look at the MCS80 manual to see
    >> that.
    >>
    >> Allison

    >
    >Correct that I stepped into something stinky, or that adoption of new
    >architecture generally followed preliminary guidelines?
    >
    >As always, Allison, your insight and citations are a treasure!
    >

    I was among the lucky when the 8008 hit and again when the 8080 hit
    So being there and paying attention I saw. Didn't hurt to save all
    preserve my docs.

    Intel drove the unified bus (bidirectional) with cleaner controls when
    the Bipolar part 8228/8238 became available. Those ideas were
    encapsulated in multibus and later the very similar PC xt.


    Allison

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