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scsi device ID assigning

Discussion in 'A+' started by Steven, Jul 25, 2008.

  1. Steven

    Steven Bit Poster

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    Hi People

    I'm currently on ProProfs.com using the test questions for A+. There has been a few questions implying there is an order when coming to assign ID's for scsi devices. This has left me a little confused as I'm reading the Mike Meyers book for study and this states there is no need to order just ensure each device has a different ID on the same adaptor.

    Could people please give their thoughts
     
  2. greenbrucelee
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    greenbrucelee Zettabyte Poster

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    What the Mike Meyers books say is correct, it does go by the position on the cable but you do not have to assign the drive IDs.
     
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  3. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    <Cough> This isn't correct. The cable plays no part in allocating device IDs. (IDE cables can do this - but not SCSI)

    Each drive has a set of links or switches to set the ID. Each drive must have a different ID.
    (I say drive here - but it could equally well be something like a scanner.)

    The *priority* on the bus depends on that ID - so you normally set the boot drive to ID 0.

    There was a version of SCSI that was supposed to do this all automatically - but I've never seen it work, and believe it is no longer in the standard.

    Harry.
     
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  4. greenbrucelee
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    greenbrucelee Zettabyte Poster

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    My mistake, I though the ID was set automatically due to where the drives started and drives ended on the cable :blink
     
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  5. Luddym

    Luddym Megabyte Poster

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    Plus, the end of a SCSI chain needs to be terminated.
     
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  6. Boycie
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    Boycie Senior Beer Tester

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    This is a feature of IDE - cable select. Avoid it if you can. :D
     
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  7. greenbrucelee
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    greenbrucelee Zettabyte Poster

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    Duh, I think I started on the beer too early. I know cable select sucks and IDE is defined by the position on the cable.

    I know the devices need terminated. I have had a bad very long week. At least I don't back till Monday which makes a change.
     
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  8. Sepher

    Sepher Bit Poster

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    I agree it doesn't matter where on the chain it is. It also needs to be terminated at both ends, even if you have SCSI devices externally connected.

    The pioririty for 8 bit SCSI is 7, 6, 5, 4, 3, 2, 1, 0 where 7 is highest.

    The pioririty for 16 bit SCSI is 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8 where 7 is highest again.

    Scsi ID 7 is normally allocated to host adapters.

    You can set, as said, SCSI ID via DIP switches, jumpers, or dials.

    Multiple devices can have the SAME SCSI ID by using a logical unit number. :D

    I just can't believe how many SCSI standards there are :blink
     
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  9. supernova

    supernova Gigabyte Poster

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    The comptia covers older and original standards. Because it is often (LOL) still seen today and its good to know for backward compatibility ...... and also theory.

    The fact is i haven't really had to worry about ID's and terminating for a while now its automatic in modern controllers and drives.

    Saying that people still stick to the old 0 boot, 7 host and 4 CD/DVD assignments.

    I have used SCSI since 91 starting off with bustmaster controllers (any older members remember them?) before moving over to adaptec . I remember at school saving dinner money to go towards 2 SCSI drives. :oops: ... i even had a SCSI scanner.

    Andi
     
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  10. Sparky
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    Sparky Zettabyte Poster Moderator

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    Yup, SCSI isn’t as difficult as what it used to be. Generally the SCSI controller will hand out the SCSI Ids to the devices on the SCSI cable now.

    Some of the older SCSI controllers I have had to set the SCSI ID with the jumpers on the HDs.
     
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