U320 Cable Lengths

Aloisi, Paul paul_aloisi at ti.com
Mon Aug 25 03:56:01 PDT 2003


* From the T10 Reflector (t10 at t10.org), posted by:
* "Aloisi, Paul" <paul_aloisi at ti.com>
*
Ultra320 SCSI requires different cables and spacing than Ultra160 or Ultra2
SCSI. Unfortunately the PIP and SSM standards were not ready when Ultra320
was released. The successful multidrop cables that I have used for Ultra320
and Ultra640 testing were based on 5 1/4 inch spacing between flats with the
connectors placed at 2 or 3 flats apart, alternating the spacing. The 9.85
inch spacing used for Ultra160 and Ultra2 are actually a filter for the
Ultra320 and Ultra640 speeds. Most backplanes had to be redesigned for
different spacing, many of the backplanes had the same problem as the
cables, creating a filter for the Ultra320 and Ultra640 speeds.

What helps a lot of these system is to drop the terminator impedance, if you
used the SPI-5 terminator for Ultra320 systems and drop the impedance to
minimize the reflection from the terminator, this will help a lot on
backplanes with close spacing. 

The SPI-5 standard refers to both PIP and SSM-2 standards, these are all
published standards. If you used the SPI-5 documentation for Ultra160,
Ultra320 and Ultra640 systems, you will get much better results.
Unfortunately the programmable terminator, PIP and SSM-2 were not ready when
we released SPI-4, which first documents the Ultra320 speed. 

Thank you,
Paul Aloisi

-----Original Message-----
From: Leon Toh [mailto:tltoh at attglobal.net] 
Sent: Monday, August 25, 2003 2:03 AM
To: t10 at t10.org
Subject: Re: U320 Cable Lengths

* From the T10 Reflector (t10 at t10.org), posted by:
* Leon Toh <tltoh at attglobal.net>
*
U320 have been a real nightmare and it still is for many of us. My 
experience here are mostly with U320 HDD in RAID environment.

Once I have 4 or more U320 HDD attach to a multi-drop cable I start having 
nightmares. At most times with 4 U320 HDD I find that the HBA will 
negotiate reliably for most of the HDD at U160. If I wanted U320 
reliability than I have to take a number of factors into consideration. For 
me I find that the type of multi-drop cable and cable quality happens to 
play a major role besides HDD firmware when comes to U320 negotiation 
reliability.

As you mention that the specs specifies that for a multi-drop configuration 
maximum allowed cable length is 12.5 m. To me this is virtually impossible 
going by the number of issues which I have encountered. By the way we are 
also having issues when comes to hotswap passive backplane using U320 HDD's 
also.

Majority of the issues I'm experiencing here I would say they are mostly 
noise related. Reason I got to this conclusion was mainly the fact that 
there is no consistency at all and in some instances using the same 
multi-drop cable, HDD and HBA but install into a completely different 
Server / Workstation sometimes it work reliably for me.

I hope that HDD vendor's won't be releasing a U640 parallel version when 
the specs is finally finalise. Otherwise I can imagine the amount of issues 
we would face when parallel U640 version becomes available.


Best Regards,
Leon

At 09:25 AM 23/08/03, Kevin D Butt wrote:

>I have a question about real-world cable lengths in use in Ultra 320 
>environments.  SPI-5 lists the cable lengths at 25 m for point-to-point 
>and 12,5 m for multi-drop.
>Is there anybody out there who is actually using 25 m cables in point to 
>point without any issues?
>Is there anybody out there using 12,5 m cables with mulitple devices 
>attached without any issues?
>How many devices have successfully been used on one cable?
>
>Thanks,
>
>Kevin D. Butt
>Fibre Channel & SCSI Architect, IBM Tape Microcode,
>6TYA, 9000 S. Rita Rd., Tucson, AZ  85744
>Tie-line 321; Office: 520-799-5280, Lab: 799-5751, Fax: 799-4138, Email: 
>kdbutt at us.ibm.com

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