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Switchable SCSI terminator schematic

Discussion in 'Electronic Design' started by NoSp, Feb 29, 2008.

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  1. NoSp

    NoSp Guest

    Where can I find a schematic for a terminator (SCSI-1) which can be
    switched on and off?

    The idea is to add an external SCSI connector to my system which will
    either be unused (and therefore must be terminated) or connected to some
    external SCSI device (and must be terminated).

    Instead of pluggin/unplugging an external terminator I was thinking of a
    more elegant solution where I have a switch to turn termination on or off.

    Here's a block diagram I've made:
  2. Fred Bloggs

    Fred Bloggs Guest

    There are such things as repeaters which provide the termination for
    both directions, and these are low cost:
    The terminators are even cheaper, may be better to just bring a length
    of cabling out and plug a terminator into the end of that, then use an
    IDC tap to stub out at 10cm max for another device, the stub can be left
    Elegance has more to do with using what's readily available for the
    application than wistful wishing for something you're incapable of building.
  3. Guest

    Here is what you want

  4. whit3rd

    whit3rd Guest

    There are ICs that implement a good SCSI terminator; just buy them,
    don't try to reinvent. UCC5672 or DS2119 are available at DigiKey.

    Two problems are implicit in your stated requirements

    (1) the SCSI-1 specification called for 220/330 ohm 5% terminator
    and connection to +5V, with wide error margins. Later SCSI
    made it 110 ohm 1% terminator and connection to regulated 2.85V.
    This 'active' termination is what logic-switchable termination ICs
    it's a free upgrade from the old termination.

    (2) many SCSI devices have differential (either low voltage or RS-485
    high voltage differential) inputs/outputs and must be disabled, using
    feature of the SCSI standard called 'differential sense', if a non-
    (i.e. 'single-ended', or SE) device is attached, Your terminator
    IS a SE device, and should only be used when the 'differential sense'
    implemented properly (i.e. you really want to adopt this part of the
    subsequent standards after SCSI-1).
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