SCSI Bus Problem?

From: Tothwolf <tothwolf_at_concentric.net>
Date: Mon Feb 11 22:46:20 2002

On Mon, 11 Feb 2002, Julius Sridhar wrote:
> On Mon, 11 Feb 2002, Tothwolf wrote:
>
> > Original - narrow, 5Mb/s [used by lots of older devices]
> > Fast - narrow 10Mb/s [very common]
> > Ultra - narrow 20Mb/s [very common]
> > Wide - wide, 20Mb/s [common for a short time]
> > Ultra-Wide - wide, 40Mb/s [very common]
> > Ultra2 - wide, 80Mb/s [never really caught on]
> > Ultra160 - wide, 160Mb/s [starting to become common]
> > Ultra320 - wide, 320Mb/s [not common yet]
>
> Ok. First of all, it's MB/s, rather than Mb/s.

*shrug*

Actually, it would have been better to type the whole thing up with 'MHz'
and bus width, instead of just MB/s, but I was in a hurry. Oh well, use
Google.

> Second, MB/s is not a good way to describe the bandwidth of the bus
> for technical reasons. A fast/wide 20MB/s bus looks *nothing* like an
> ultra 20MB/s bus. This is so because fast/wide is a 10MHz 16-bit bus
> and ultra is a 20MHz 8-bit bus.

I didn't imply that a "fast-wide" bus was anything like the "ultra" bus.

> Just because they have the same bandwidth doesn't mean anything. An
> ultra device will run at 10MB/s on a fast/wide bus and a fast/wide
> device will run at 10MB/s on an ultra bus. Both busses are *capable*
> of 20MB/s.

Of course the ultra device would run at 10MB/s, a fast/wide bus runs at
10MHz, and the ultra device is only 8 bit instead of 16.

> If you have an ultra device and an ultra/wide device on an ultra/wide
> bus, the ultra device will run at 20MB/s and the ultra/wide device
> will run at 40MB/s *simultaneously* **on the same bus**.

I'm not disputing that.

> > (info from memory, might be incomplete, inaccurate, etc, etc)

Uhm, I did put a disclaimer in here...

> > There are also Differential versions of Fast, Ultra, Wide, and Ultra-Wide.
> > These use a "high voltage" (+-12VDC IIRC) signaling that is *NOT*
> > compatible with standard devices. You will literally fry any non HVD
> > devices if you connect a HVD drive to the same bus. Ultra2 and newer have
> > a Low Voltage Differential bus, I'm not sure if there is a HVD
> > specification for those.
>
> Signalling voltages are irrelevant to this discussion.

Actually, I think not. With the way this thread has been going, I think it
is quite a good idea to mention the differences between "standard", HVD,
and LVD signaling.

-Toth
Received on Mon Feb 11 2002 - 22:46:20 GMT