SCSI termination question

[Dilitante1]

Take a look here yet?

http://www.scsifaq.org/

or

http://www.scsi.com/White.htm

http://www.scsi.com/scsiWorld.htm

my other links arent working/or vendor specific atm.....but some of it is in plain english...

-Dil

[Patrick]

Some good news...

But that is a later step, first we'll try to maximize the disks' performance.

Jörg, I'm working on it now. I'm reading over lots of material before I ask any more questions. One bit of good news- The HP scanner which was connected to the SCSI interface will also connect using USB, so that's an option I didn't realize I had earlier if I need it.

Dil, thanks for the links. Lots to read!

[xortam]

Regarding LVD ... I referred to LVDs when using the Ultra2 connector and I stated that I assumed that the drives were LVD. We need to go back to the previous configuration of placing all the devices on the SE segment now that we know that the drives are Ultra Wide. Patrick ... do you have a URL for the specs on your scanner? Basically we need to know the SCSI level (50 or 60 pin connector? wide or narrow? speed?). I couldn't find your model on the HP site. I did notice that the 6000 series supported USB as well so, as you say, that is an option. Lets try to get in on the SCSI bus though as that should get you some better performance. It looks like we may have to deal with high byte termination as was previously mentioned. What are the lengths of your ribbon cable and your scanner SCSI cable?

[Patrick]

Still plodding along...

This is turning into a long thread. I just want to repeat something I mentioned much earlier. This is not my computer which helps explain why I know so little about it's SCSI drives and scanner.

xortam, the scanner is an HP 6200c. The number I gave earlier (C6270A) was actually a part number which is different than the model number. For some reason HP does not have manuals or specs at their site, but I found the following at the HP site which may be of some help.

http://www.hp.com/cposupport/scanners/support_doc/nps00070.html

There are some instructions there regarding configuring the host adapter settings, but I note it also states this:

HP has no information on the effect that these changes may have on other devices attached to the Adaptec SCSI card.

Obviously having two drives on the same cable would alter the configuration. I removed and measured the length of the internal ribbon cable and it's 33 inches. I have no idea how long the external cable is that goes to the scanner. (I don't have it.) Do I need to find out? Is there a definite performance loss if we simply make use of the USB connection?

I haven't finished reading over all the links and information I've been provided with yet. I'll be checking back here later...

[xortam]

I saw that "Connection Issue " page and was going to post that for you. Glad you found it. The SCSI on the P2B-xS MBs is equivalent to Adaptec's 2940U2W card. Check out the 2940U2W installation guide for more info on connections. I believe you're scanner is Fast SCSI using a HD 50-pin connector (HD is High Density). Unfortunately, the Wide SCSI connector on the MB is always terminated. Otherwise, you could place the host controller in the middle of the bus and use the Wide SCSI connector for your drives and the 50-pin Fast SCSI connector externally to your scanner.

Here's a couple ways I think will work:
The obvious choice is to have your drives connected to the Wide SCSI connector using a 68-pin ribbon cable with a 68-pin external connector (Mounts in card slot opening. You should have extra non-card slots for this). You would then plug in a 68-pin-to-HD 50-pin converter (w/ hi byte terminator) to the outside of your case into this new 68-pin external connector. Then connect your HD 50-pin cable to your scanner which I believe is self-terminating. There should be no terminators on your drives. The other end of your bus (internal and connected to the host controller) is always terminated.

Another way that may work would preclude you from having to buy the 68-pin-to-HD 50-pin converter ... Try connecting your drives to the Ultra2 connector with the last drive being terminated. The LVD segment should negotiate down to support the slower speed of your drives. Now connect a 50-pin ribbon cable with HD 50 external connector to your Fast SCSI 50-pin connector on the MB. You can now just simply connect your HD 50 scanner cable to this new HD 50 external connector. Remember that now you are using the segmented bus and you need to deal with four termination points. The LVD segment will have to be terminated in the BIOS and also at the last drive on the 68-pin ribbon cable. The SE segment will have to also be terminated in the BIOS but the scanner should self-terminate at the other end of the bus segment.

The length problem is that SE devices are limited to a total bus length of 1.5m (about four feet). You need to total the lengths of the 68-pin ribbon cable, the 50-pin ribbon cable, and the 50-pin external cable in this last example.

P.S. AFA performance difference ... the HP site said its about 50 percent faster on SCSI. I'll look up the text for you tomorrow if you like.

Also, you'll need to insert a terminator plug to the external connector if you decide to disconnect the scanner.

[Patrick]

Slowly getting there...

The obvious choice is to have your drives connected to the Wide SCSI connector using a 68-pin ribbon cable with a 68-pin external connector (Mounts in card slot opening.)

Would you believe that's how it already is? I kid you not. However I have no idea what kind of cable (or converter) my friend plugs into this external connector.

There should be no terminators on your drives.

Would you believe there were terminators on BOTH drives when I opened up the case? I kid you not.

Does this information simplify things a little?

Interestingly enough, this brings me back to my very first post which asked:

1) With everything (including scanner) connected, should the termination jumpers on both drives be removed?

2) When the scanner is NOT connected (as it is now because I don't have it here), does that make a difference whether or not the drive farthest from the "host" has its termination jumper removed?

I'd say that the answer to #1 is YES and the answer to #2 is also YES, the termination jumper needs to be put on the drive farthest from the host adaptor when the scanner is removed (or a terminator plug needs to be used on the external connector). Have I got it right?

Of course I'm still not 100% sure how to configure the host adapter settings for these Barracuda (ST118273W) drives yet, but I'm working on it!

[xortam]

You've got it right!

A correction on the length constraint. On your setup you can go up to 3m, not 1.5m. 1.5m seemed too short to me and I double checked this morning and sure enough ... it was wrong. Check here for the length constraints. I found that site to be prettty good. There are plenty of SCSI sites on the web but I think you've got enough now.


/edit/
Too many 'o's in "too short too me"
/end-edit/

[VJ]

xortam:
Very good explanation on your previous post ! There is nothing I could possibly add.

I now have a question concerning your link w.r.t. the length limits. My SCSI controller is an Adaptec 2940UW, which is an Ultra Wide controller. According to the details of the Ultra Wide protocol

http://www.pcguide.com/ref/hdd/if/scsi/modes_UltraWide.htm

only 8 devices are possible (except when using HVD). I know my controller is not HVD, but the bios supports 16 devices. What is the explanation ?

Thanks,


Jörg

PS: The 1.5m cable limit counts when more than 4 devices are connected, which is why I stated 1.5m (this is the limit I had to respect in my system). I didn't know that with 4 (or less than 4) devices the spec allows for 3 m.

[xortam]

VJ ... I'm glad you like the explanation. I try to help.

AFA UW SCSI device limits ... I think 16 is correct. I'll check it out later after I get back from walking the dog ...

[xortam]

VJ ... The best I can tell is that UW SCSI supports up to 16 devices if all of the devices support LVD signaling. The installation guide for your adapter states that it will support up to 16 devices. You may need to contact Adaptec to get clarification on this.

[Patrick]

If it's not one thing it's another...

I've had to temporarily put on hold my efforts with this computer as I've developed a serious problem with my own computer. (One of the partitions on the boot drive has "disappeared".) Anyways, I'll give a report when I resume activities on all this SCSI stuff. Thanks for all your help guys.

[VJ]

xortam:
I know the controller supports 16 devices, but this contradicts with the mentioned link :

http://www.pcguide.com/ref/hdd/if/scsi/summary.htm

and also with the more detailed UW-information (on the same site) :

http://www.pcguide.com/ref/hdd/if/scsi/modes_UltraWide.htm

I just rechecked the specs of Patricks harddisk (just as an example of an UW-drive that doesn't support LVD)

http://www.seagate.com/support/disc/scsi/st118273w.html

and

http://www.seagate.com/support/disc/specs/scsi/st118273w.html

Apparently, it supports 16 different SCSI-id's, so I doubt LVD-signaling-capabilities of a devices have anything to do with being able to go up to 16 devices. They need to be UW to be able to select id's over 7, but I'd say that's about it.

I think it are such contradictions that make SCSI a "difficult" standard (one site mentions A, another mentions B).


Jörg

[xortam]

I appreciate that there are contradictions and/or confusions. Also, remember that sometimes manufacturers produce products that don't agree with industry standards and specifications. I've seen many sites that concur with the information that is published on the PC Guide site. I've also seen UW products such as your SCSI adapter that support 16 devices. I was trying to find information that would indicate the bus length constraints when using all 16 devices. I'll show you the information I found that led me to the conclusion in my previous post ...

Adaptec states in one of their knowlegebase articles that UW supports up to 16 devices. The installation guide for your controller confirms this.

The STA site shows a chart which lists UW as supporting 16 LVD or HVD devices. Note that they state:

LVD was not defined in the original SCSI standards for this speed. If all devices on the bus support LVD, then 12-meters operation is possible at this speed. However, if any device on the bus is singled-ended only, then the entire bus switches to single-ended mode and the distances in the single-ended column apply.

This is what led me to post my previous comment.

So why do we see non-LVD/HVD UW products supporting 16 devices? Non-compliance? I think its probably due to a change in the standards. I was trying to find a copy of the spec on the Net (ANSI standard X3.277-1996) but am unable to access an on-line version. I can look over the spec if anybody can post a reference to an on-line copy. This may need a call to Adaptec or a post to the SCSI NG in order to flush this out. I'll keep looking ...

[xortam]

I still wasn't able to get hold of the standard (gots to pay for that) but I did finally manage to get into the T10 ftp site to download the latest drafts. I downloaded the SPI draft (became ANSI standard X3.277-1996) and the SPI-2 draft (became ANSI standard X3.302-1999 which superseded SPI). I'll pour through these later and see if I can gleam the info out of them. But first, I've got to go off and walk the dog again ...

[xortam]

The best that I can determine is that the higher device count requires higher spec equipment (beyond minimum requirements established in the SPI-2 spec) that will allow for closer device spacing. The spec calls for a maximum 10 cm stub length with a minimum inter-device spacing of three times the stub length (30 cm for worst case).