This is part of The Pile, a partial archive of some open source mailing lists and newsgroups.
Date: Sun, 20 Feb 2000 13:30:17 -0700
From: "D. Stimits" <stimits@idcomm.com>
Subject: Re: SCSI devices on one bus
Keith Rhodes wrote:
>
> I seem to remember a while ago having read that is devices
> of varying speeds are connected to the same SCSI bus, all
> the devices on that bus run at the speed of the slowest
> device.
>
> 1. Is this in essence true?
Yes.
> 2. If a SCSI card has an internal connecter and an external
> connecter, are the external devices on the same bus as the
> internal devices?
It depends on the hardware. I know on the integrated Adaptec 7890/7895
boards that have both a 50 pin and larger pin connector, the 50 is
sitting on the faster uw or u2w bus; adding something slow to the bus
will slow all on it. During bootup, you should see a bios or other
scsi detection routine, and most likely it will tell you which chain
it is on. On my dual chain, I keep only slow devices on one chain,
including a SCSI cd rom, and all the faster stuff on the other chain.
>
> If this is the case, then surely it is better to have two
> SCSI cards... a slower (ergo cheaper?) card for slow devices
> e.g. CD burner, tape streamer, and a second, fast card for
> hard disks.
Correct. Otherwise you lose performance.
>
> KR.
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From: ericp@earthlink.net (Eric Potter)
Subject: Re: SCSI devices on one bus
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[snip]
>
>>
>> If this is the case, then surely it is better to have two
>> SCSI cards... a slower (ergo cheaper?) card for slow devices
>> e.g. CD burner, tape streamer, and a second, fast card for
>> hard disks.
>
> Correct. Otherwise you lose performance.
>
Strictly speaking, I believe you'll only lose performance if the slower device
is in use. All good SCSI adapters will feature auto disconnect, which as the
name implies, will electronically disconnect an idle device from the SCSI chain.
Try to figure out how much you will be using these extra devices to determine
if it is worth it for you to buy a second card.
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From: stu@c49395-a.wodhvn1.mi.home.com (Stuart R. Fuller)
Subject: Re: SCSI devices on one bus
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Keith Rhodes (klrhodes@club-internet.fr) wrote:
: I seem to remember a while ago having read that is devices
: of varying speeds are connected to the same SCSI bus, all
: the devices on that bus run at the speed of the slowest
: device.
:
: 1. Is this in essence true?
NO, IT IS NOT TRUE!
The controller will query each device and run the bus at the fastest speed
that the device and controller can do. For slower devices, it'll run at the
slower device's speed. For faster devices, it'll run the bus at the faster
device's speed. The bus runs at the fastest speed that each device will run
at.
Now, if you have a stupid tape drive that won't disconnect from the bus while
it's rewinding, then everything stops while it rewinds.
Stu
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Date: Mon, 21 Feb 2000 18:37:58 -0700
From: "D. Stimits" <stimits@idcomm.com>
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Subject: Re: SCSI devices on one bus
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"Stuart R. Fuller" wrote:
>
> Keith Rhodes (klrhodes@club-internet.fr) wrote:
> : I seem to remember a while ago having read that is devices
> : of varying speeds are connected to the same SCSI bus, all
> : the devices on that bus run at the speed of the slowest
> : device.
> :
> : 1. Is this in essence true?
>
> NO, IT IS NOT TRUE!
>
> The controller will query each device and run the bus at the fastest speed
> that the device and controller can do. For slower devices, it'll run at the
> slower device's speed. For faster devices, it'll run the bus at the faster
> device's speed. The bus runs at the fastest speed that each device will run
> at.
>
> Now, if you have a stupid tape drive that won't disconnect from the bus while
> it's rewinding, then everything stops while it rewinds.
>
> Stu
There is another possible complication. Sometimes there is also a
different electrical specification, not just speed. In particular, low
voltage differential (LVD) drives may function on a chain with other
hardware that is not LVD, but even if those drives disconnect
*control* of the bus, I have to wonder if maybe LVD can be achieved by
any device on the bus (the other hardware is still on the chain, even
if it isn't controlling it). I'd be interested if anyone knows for
sure on that subject, since I'm doing some guessing.
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===
Subject: Re: SCSI devices on one bus
From: Johan Kullstam <kullstam@ne.mediaone.net>
Date: Tue, 22 Feb 2000 03:53:55 GMT
"D. Stimits" <stimits@idcomm.com> writes:
> "Stuart R. Fuller" wrote:
> >
> > Keith Rhodes (klrhodes@club-internet.fr) wrote:
> > : I seem to remember a while ago having read that is devices
> > : of varying speeds are connected to the same SCSI bus, all
> > : the devices on that bus run at the speed of the slowest
> > : device.
> > :
> > : 1. Is this in essence true?
> >
> > NO, IT IS NOT TRUE!
> >
> > The controller will query each device and run the bus at the fastest speed
> > that the device and controller can do. For slower devices, it'll run at the
> > slower device's speed. For faster devices, it'll run the bus at the faster
> > device's speed. The bus runs at the fastest speed that each device will run
> > at.
> >
> > Now, if you have a stupid tape drive that won't disconnect from the bus while
> > it's rewinding, then everything stops while it rewinds.
> >
> > Stu
>
> There is another possible complication. Sometimes there is also a
> different electrical specification, not just speed. In particular, low
> voltage differential (LVD) drives may function on a chain with other
> hardware that is not LVD, but even if those drives disconnect
> *control* of the bus, I have to wonder if maybe LVD can be achieved by
> any device on the bus (the other hardware is still on the chain, even
> if it isn't controlling it). I'd be interested if anyone knows for
> sure on that subject, since I'm doing some guessing.
see <URL:http://www.scsifaq.org/>
the bus is *either* in LVD *or* in single ended (SE) mode. if any
device is SE, then the whole bus is operated in SE.
once in SE mode, you can mix fast and slow devices without slowing
each other down. they don't have to talk to each other, just to the
controller.
for example, with a UW controller, a UW hard drive and a fast, narrow
(FN) CD-R, the hard disk can still talk at 40MB/sec despite the
presence of the 10MB/sec limited CD-R.
however, when they are using the bus, slow/narrow devices occupy
more time on the bus and can cut into the total throughput of the
bus. in the above case, the CD-R will occupy 4 times as many time
slots as the hard disk to move the same amount of data. (i dislike
referring bandwidth because the scsi bus is *not* frequency sliced,
but time sliced.)
--
J o h a n K u l l s t a m
[kullstam@ne.mediaone.net]
Don't Fear the Penguin!
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From: "Robert W. Cunningham" <rcunning@acm.org>
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Subject: Re: SCSI devices on one bus
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"D. Stimits" wrote:
> There is another possible complication. Sometimes there is also a
> different electrical specification, not just speed. In particular, low
> voltage differential (LVD) drives may function on a chain with other
> hardware that is not LVD, but even if those drives disconnect
> *control* of the bus, I have to wonder if maybe LVD can be achieved by
> any device on the bus (the other hardware is still on the chain, even
> if it isn't controlling it). I'd be interested if anyone knows for
> sure on that subject, since I'm doing some guessing.
That's right. A SCSI bus is capable of LVD operation ONLY when ALL devices on
that cable are capable of LVD operation. Otherwise, LVD devices fall back to
normal bus drive circuitry. There is the odd case where one controller talks to
two cables of differing width or impedance, and if the controller terminates and
drives each cable separately, then LVD and non-LVD devices can share the same SCSI
address space, but not the same cable. The Diamond FirePort 40 uses this trick to
talk to devices on separate 8-bit and 16-bit cables (though it is not LVD
capable).
-BobC