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Date:	Sun, 31 Aug 2003 11:09:18 PDT7
From:	Steve Carter <scarter@vcnet.com >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: What SCSI adapter to choose !?

Content Type: text/plain

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Most SCSI BIOSes allow you to specify the bus speed on a target-by-target
(SCSI ID) basis. This permits one to use 5mB/s, 10mB/s, 20mB/s devices
and 40mB/s devices on the same single-ended (SE) bus. I have no experience
yet with Low Voltage Differential (LVD), but it seems fair to assume
that this BIOS feature would continue with LVD.

The most important point is that if you mix SE and LVD devices
on the same bus, the bus reverts to SE and everything runs at
some SE speed. Your super-duper ultra2/160/320 hard drives will
operate as SE, but at a far-less-than-optimum transfer rate.
It might be an acceptable interim solution for someone, like me,
who has yet to buy his new dual LVD Host Adapter.

The 64-bit PCI bus was originally intended to be backwards-
compatible with 32-bit PCI used on MBs today. It is reasonable
to expect most 64-bit HAs to be 32-bit compatible. However, dual
U320 HAs have a SCSI bandwidth of 640mB/s and therefore use the
133MHz, 64-bit PCI-X interface at 1024MB/s. It's important to
verify both 32-bit compatibility and fit before spending real money.

I have received replies back from LSI support, but not from Tekram.
Perhaps a phone call is required, if the info is not specifically
stated in the documentation. The Tekram web site does state
compatibility with 32-bit PCI for their U160 HAs. The LSI Logic
LSI 21040, and LSI 22915A documentation indicates 32-bit compatibility.
The LSI 21003 is 32-bit, as is the Adaptec 29160N and 19160. Adaptec
claims 32-bit compatibility for their 64-bit AHA-29160 & 39160 HAs.

There were errors in the Tekram section of my previous post,
but corrected below:

Tekram (uses LSI Logic SCSI chips):

DC-390U3D (dual channel, 30 devices max):
2x U160 LVD channels; 2x 68-pin VHDCI external,
1x 50-pin internal; 64-bit PCI

DC-390U3W (dual bus, 15 devices max):
1x U160 and 1x SE channel; 1x 68-pin external LVD, 1x 68-pin
internal SE, 1x 50-pin internal SE; 64-bit PCI

-- Steve

+++++++++++
On 8/30/03, Steven Levine wrote, in part:
>On 08/29/03, Steve Carter said:
>
>>Dual channel SCSI host adapters make a LOT of sense for backwards
>>compatibility reasons. Although SCSI is both backwards and (somewhat)
>>forward compatible, the HA will operate the bus at the lowest common
>>denominator (speed) of whatever is on the bus.
>
>This is not quite true. As long as the devices are properly designed, for
>a given termination type (LVD/DE) the individual devices will operate at
>their rated max speed. Some of the slower devices might have touble
>negotiating transfer rates, so the HBA BIOS provides settings to handle
>this.
>
>>If your motherboard doesn't have 64-bit PCI slots, you can still use
>>a 64-bit PCI card provided you have clearance for the additional
>>connector length.
>
>This true for some of the 64-bit cards, but not all.
>Some of the U320 HBAs claim to be 64-bit only.
>
>Steven
>--

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