[3882] in linux-scsi channel archive
Re: changing host ID of ncr53c810?
daemon@ATHENA.MIT.EDU (Richard Waltham)
Sat May 9 17:03:00 1998
From: Richard Waltham <dormouse@farsrobt.demon.co.uk>
To: h.milz@seneca.muc.de
Date: Sat, 9 May 1998 21:48:29 +0100 (BST)
Cc: linux-scsi@vger.rutgers.edu
In-Reply-To: <m0yY8Rb-000354C@seneca> from "Harald Milz" at May 9, 98 02:09:00 pm
Harald Milz wrote:
>
> Richard Waltham (dormouse@farsrobt.demon.co.uk) wrote:
> > #ifndef SCSI_NCR_MYADDR
> > -#define SCSI_NCR_MYADDR (7)
> > +#define SCSI_NCR_MYADDR (6)
> > #endif
>
> In order to allow for setting the ID without patching the driver, I strongly
> suggest a kernel/module command line parameter to set the host adapter SCSI
> ID. This will be important as soon as some shared SCSI bus high
> availability software package starts to do real work. Since the SCSI ID
> will not change very frequently, it should suffice to set it via the
> adapter BIOS setup routines. But apparently not all adapters allow this
>
No problem here but as you say if its not changed frequently then patching
the driver is a reasonable way when its not available as an option from the
bios. On the card in question the host id can only be changed currently by
patching the driver.
> > Apart from the host ID there are other problems that could occur but thats
> > another story. But you should be ok if you only access the drives from one
> > system at a time.
>
> No, there's a catch. The SCSI bus needs to be terminated at the far ends
> ONLY. If you put the SCSI peripherals "between" the adapters, you need to
OK
> disable bus termination on the adapters and use a Y cable
Pardon? The SCSI bus needs terminating at each end. If the adaptors are at
each end then they need terminating or you need external terminators.
There is nothing magic about a scsi host adaptor. As far as the scsi bus is
concerned it is just another scsi device. The same rules for terminating
buses apply whether you have only one host adaptor or several. The bus needs
terminating at both ends.
Y cable? A REALLY BAD IDEA!
> (http://sunsite.unc.edu/pub/Linux/ALPHA/linux-ha/High-Availability-HOWTO-7.html)
> to make sure the bus isn't unterminated at one end if the machine at this
> end is switched off. You can also put the SCSI host adapters in the
Host adaptors should supply terminator power to the scsi bus on the
termpower lines. If a device with built in terminators is powered off the
terminators will be maintained with termpower from the scsi bus. This
applies to another host adaptor as well as any other device.
It is a requirement of at least SCSI 3 that terminators receive power from
the termpower lines and that they maintain operating within spec as long
as at least one device is supplying termpower. I would expect most current
devices to meet these requirements.
The host adaptors I have can have their terminators supplied with power from
the termpower line of the SCSI bus - thats what termpower is for after all -
to supply power to terminators, either an external terminator on the end of
the cable or built into a device even when power to the device is turned
off. This applies to host adaptors as well as any other device - disk, tape
etc. that may have a built in terminator and is on the end of the bus.
> "middle", disable termination on the adapters and attach terminators at the
> far ends of the SCSI bus. Any way the SCSI host adapters need to have
> termination disabled. More is documented in the HA HOWTO pointed to above.
>
NO! Not "any way"!
The host adaptors do not need their termination disabling "any way". If
they are in the middle of the bus - yes. If they are at the ends of the bus
but external terminators are used on the ends of the cable - yes. If they
are on the ends of the bus without external terminators then the terminators
on the host adaptors need to be enabled!
The host adaptors need terminating if at the ends of the bus and if external
terminators are not used. They need to supply termpower to the scsi bus to
power the terminators at each end of the bus so, if the device at the end of
the bus, if it has a built in terminator and the device is powered off, the
terminator remains powered, whether it is a host adaptor or any other
device.
Looks like the howto may need updating.
ftp://ftp.symbios.com/pub/standards/io/t10/drafts/spi2/spi2r20a.pdf sections
4 and 7 may help.
Bye for now
Richard
>
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--
Richard Waltham | Work: richard@digtalinterface.demon.co.uk
At home in | Home: dormouse@farsrobt.demon.co.uk
Southampton UK | 100421.1276@compuserve.com
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