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Help understanding SCSI connections, channels, Buses  and Termination

Posted on 2010-09-02
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We have a server which has an onboard SCSI controller and an Adaptec 2100S PCI SCSI card installed. From the SCSI card is a cable with 6 connections. The second connection on the cable is connected to a disk RAID Array which contains 5 disks via a SAF-TE hot swap backplain.

Can you confirm that the system sees the onboard SCSI controller completely seperately from the SCSI PCI card? That is they can both have their own sequence of SCSI IDs. Would they be on seperate BUSES and can each BUS have is own sequence.

My understanding is that everydevice (including cards) would have their own SCSI ID. Is the following correct/possible?

Bus A SCSI ID 1 - Onboard SCSI Controller
Bus A SCSI ID 2 - Tape Drive

Bus B (PCI Card) SCSI ID 1 - PCI SCSI Card
Bus B SCSI ID 2   - RAID Hotswap BackPlain
Bus B SCSI ID 3 - SCSI Drive 1 in RAID
Bus B SCSI ID 4 - SCSI Drive 2 in RAID
Bus B SCSI ID 5 - SCSI Drive 3 in RAID
Bus B SCSI ID 6 - SCSI Drive 4 in RAID
Bus B SCSI ID 7 - SCSI Drive 5 in RAID


My final question is about termination. If the first cable connector is connected to the PCI Card (with auto termination) and the Second cable connector connected to the RAID backplain, should the terminator be at the third cable connctor possition or is it ok to have the connector at the very end of the ribbon with entry connectors? At the moment is is at the very end of the cable and clips on so that it can not be easily removed.

Thanks for your help
 
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Question by:EICT
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helpnet earned 420 total points
ID: 33585903
The SCSI configuration you have listed is possible (but can't say it is correct without seeing the computer).  

The onboard SCSI controller acts like another scsi card and its bus is totally separate to that of the SCSI card

Each bus is it's own scsi chain, and should be terminated.

Termination must be present at two and only two positions on the SCSI bus, at the beginning of the bus, and at the end, and must occur within 4 inches of the physical ends of the SCSI bus. Often, the host will be installed at one end of the bus, and will provide one of the two terminations required.
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by:helpnet
helpnet earned 420 total points
ID: 33585947
So technically if the end of the scsi cable is more than 4 inches from the raid backplane, it should be moved.

(and yes scsi cards/controllers do have their own SCSI ID)

For performance reasons it is usually recommended for a Raid array, that the Raid array is the only thing on that SCSI bus.  If for example a TAPE drive is added to the same SCSI bus as the RAID array, it could dramatically decrease the access speed for the RAID array.  This is why most servers have more than one scsi controller.

More infor on SCIS termination
http://www.scsita.org/aboutscsi/SCSI_Termination_Tutorial.html
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by:David
David earned 80 total points
ID: 33585951
Yes, above is possible, and even better ... reasonable.  You want to put the Tape on a separate channel, which you did.   SCSI ID #2 is for the SAF-TE target device to be accurate, not for the backplane.  Helpnet's answer regarding termination is accurate.

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Author Comment

by:EICT
ID: 33585956
Hi Helpnet, when you say end to you mean end of the chain (i.e in the 3rd connector) or end of the cable in the 6th connector.

Thanks for the clarification.

Mattt
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by:helpnet
helpnet earned 420 total points
ID: 33585995
The end of the chain is the last device with a SCSI ID.. the Raid array is at the end of the chain, so I would put the terminator on the 3rd connector.
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Expert Comment

by:helpnet
ID: 33586020
You will probably find the tape drive has a jumper setting you can use to terminate the first SCSI chain. (Bus A)
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