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SCSI Controllers

Posted on 2001-08-11
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What exactly does a SCSI/IDE controller do? Is is essential if you have a SCSI/IDE hard drive?

Zargonog
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Question by:Zargonog
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by:SysExpert
ID: 6376313
A SCSI controller is required for a SCSI hard drive , and an IDE controller is required for an IDE/ATA type drive.
Each is different.
SCSI can hable up to 7 devices ( 15 in a wide configuration ),  IDE can handle 2 devices per channel.

SCSI has the ptential to be much faster , but in home use, it does not make much difference.

I hope this helps !
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by:jjeff1
ID: 6376662
Virtually every motherboard made today has an onboard IDE controller. Only expensive ones have onboard SCSI.

SCSI places a lot of the load of disk processing onto the SCSI controller, unlike IDE, where your CPU does it. This means for Server applications, SCSI is better. You'll find that for anything that requires a lot of disk activity, SCSI is better.
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jhance earned 100 total points
ID: 6377089
>>SCSI places a lot of the load of disk processing onto the SCSI controller, unlike IDE, where your CPU
does it.


Actually, this is not true.  Both SCSI and IDE are "high-level" interfaces where the CPU just tells the controller to do something and it goes and does it.  The biggest difference between SCSI and IDE are:

1) IDE can support only two devices on a cable with a max length of about 24 inches.  SCSI can support up to 7 devices on a cable with a much longer max length.  This makes EXTERNAL SCSI setups practical.

2) IDE (in its ATA/100 form) can run at speeds of UP TO 100 M bytes/sec.  SCSI, in its LVD320 format can run at speeds of up to 320 M bytes/sec. (I think there may be faster SCSI out there or coming soon...)

3) SCSI devices can "interleave" where the operating system can tell multiple devices to start operations and they will go off and do their thing independently.  IDE doesn't permit this type of operation and each operation must begin/process/end before a new one can start.  This makes possible much higher performance when the operating system supports such interleaving.

SCSI does have disadvantages.  In general the costs of SCSI drives are significantly higher than IDE.  Why?  No good reason in my view other than what the market will accept.  The costs of building SCSI vs. IDE drives are not that different.  In most cases the hardware is virtually identical with the only change being a different circuit board.
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by:jhance
ID: 6377090
>>SCSI/IDE controller...SCSI/IDE hard drive

BTW, I've NEVER seen a SCSI/IDE controller or SCSI/IDE hard drive.  I suppose it's technically possible to make such a combo unit but I seriously doubt that it would make financial sense.  Perhaps the combo SCSI/IDE controller but certainly not the drives.
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by:Zargonog
ID: 6377828
I wasn't saying a SCSI and an IDE at once, I was just wondering what the 2 different types were and what they did.

Thanks guys!

Zargonog
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