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SCSI backwards compatibility and the cable

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June 2, 2007 7:09:30 PM

Most SCSI drives are alleged to be backwards compatible, so if one connects an Ultra320 drive to a card not capable of Ultra320, in this case, an Adaptec 2940UW, the drive will drop down to the capabilities of the card.

But, can the cable matter?

If the 2940 recognizes the drive, but errors in verifying the drive, might it be necessary to upgrade to a 68 pin cable that can handle ultra 320, even tho the 2940 does not handle Ultra320?
June 2, 2007 11:16:18 PM

Yes, the cable can matter, and possibly the drive may not be compatible.

The Adaptec 2940UW is an old card. It does only single-ended SCSI connections, using the older fast, wide, and ultra SCSI standards (5 MB/sec, 10 MB/sec, 20 MB/sec, and 40 MB/sec speeds).

Your drive must be compatible with a single-ended SCSI setup. If it's an Ultra320 drive, it may only be LVD SCSI compatible. If that's the case, the drive will not work with the 2940UW card.

If the drive is single-ended capable, you will need to use a 68-pin cable with no built-in terminator. (Most 68-pin LVD cables have a built in terminator that terminates LVD SCSI connections, but not single-ended). Then you will need to enable termination on the drive using a jumper.

Your performance in this setup will suffer, since the maximum data transfer rate the 2940UW card can do is 40MB/sec, whereas your SCSI drive is capable of way more than that.
June 3, 2007 1:14:41 PM

Thanx

The new drive has a jumper that allows forcing SE mode or, the default, allowing either SE or LVD. I tried both settings, no difference.

Cable already has 1 SE and 2 LVD drives, of course, all operate as SE. And cable is already terminated by a Quantum drive at the end of the cable.

Since the controller lists the new drive, with the correct LU number, I guess the connector/cable is OK.

I am getting an identical replacement drive this week. Everybody I talk to indicates that the Seagate ST373207LW should be backwards compatible with the 2940UW, albeit with less than full performance.
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June 3, 2007 4:26:26 PM

Ya, sounds like the drive may have a problem. Try your replacement and see.

Also keep in mind that there are cable length limitations when operating a SCSI bus single-ended. If the controller is operating in Ultra SCSI mode (20MHz, 40MB/sec), the cable length is limited to 3 feet for 3 or less devices, and 1.5 feet for 4 or more devices.

Since the addition of this drive will be a 4th device, you probably have a 1.5 feet length limitation. If your cable is longer than that, you may need to drop the SCSI speed to 10MHz (20MB/sec) to operate 4 devices.

Another option is the LSI 20160. NewEgg has it for less than $100, and you'd be operating LVD @ 160MB/sec instead of single-ended. This removes the cable length limitation as well.
June 3, 2007 6:06:09 PM

Quote:
Ya, sounds like the drive may have a problem. Try your replacement and see.

Also keep in mind that there are cable length limitations when operating a SCSI bus single-ended. If the controller is operating in Ultra SCSI mode (20MHz, 40MB/sec), the cable length is limited to 3 feet for 3 or less devices, and 1.5 feet for 4 or more devices.

Since the addition of this drive will be a 4th device, you probably have a 1.5 feet length limitation. If your cable is longer than that, you may need to drop the SCSI speed to 10MHz (20MB/sec) to operate 4 devices.

Another option is the LSI 20160. NewEgg has it for less than $100, and you'd be operating LVD @ 160MB/sec instead of single-ended. This removes the cable length limitation as well.


Thanx.

Problem solved. It was the connector on the cable. I had tried two connectors. Today, I did tumblesaults and got a 3rd connector positioned properly. This time drive worked.

The LSI 20160 would not serve as a replacement for the 2940UW, as the LSI 20160, according to the info at Newegg, does not have both 68 and 50 pin connectors. I have a SCSI CD-ROM on the narrow bus. Had a tape drive on the narrow bus, but removed that to make room for the drive.

If the 2940UW dies, I could replace it with the LSI 20160, and also replace the SCSI CD-ROM with an ATAPI CD/DVD reader, but I'd ruther not do that unless necessary.

Already ran the 2940UW's drive test, no errors, so I guess there is no problem with length. Seatools Enterprise just completed drive test with no errors.

In any case, the first drive is a WD 9100, which is SE, so all drives have to run SE.
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