On Fri, 15 Mar 2002, Andrew Basterfield wrote:

> Date: Fri, 15 Mar 2002 07:52:14 +0000
> From: Andrew Basterfield <list@lostgeneration.freeserve.co.uk>
> To: Aaron J. Grier <agrier@poofygoof.com>
> Cc: port-sparc@netbsd.org
> Subject: Re: Does anyone know what the maximum hard drive
> 
> On Thu, 14 Mar 2002 23:23:37 -0800
> "Aaron J. Grier" <agrier@poofygoof.com> wrote:
> 
> > On Fri, Mar 15, 2002 at 12:47:49AM -0500, Rafael Hinojosa wrote:
> 
> > the SS2 has 50pin connectors.  it has two of them and uses a stubby (2"
> > or so) long ribbon cable for each of them.  I'm not sure how sun gets
> > away with having three connectors from a single SCSI controller. (two
> > internal, one external)
> 
> The internal part of the chain is terminated on the motherboard, the two
> internal sockets act just like connectors on a normal SCSI cable. The SCSI
> bus frequency is low enough and the drive cable stubs are short enough to
> not to cause significant reflection and interference with operation of the
> SCSI bus. You definately should not try this with 160Mb/sec SCSI disks, or
> try to plug a long SCSI cable into a motherboard socket! The external SCSI
> port does not require termination if there are no devices present, I
> presume there is some magic on the motherboard that automatically
> terminates that end of the bus if it can't see some termination outside
> the box already.

As the day job is/was an RF (electronics) engineer, I know a little
about this :-)

Andrew is absolutely right, except that the whole system is dictatated
by the speed of the slowest device. If you have 160MHz (Mb/sec) disks
connected to a SCSI bus running at 5MHz (why?) then you only need to
treat it as a 5MHz system.

At 160MHz, the "length" of a data bit on the ribbon cable is probably
around 1 metre allowing for some velocity factor (in case there are
other RF engineers out there) and the reflections from even a short
(say 10cm) length of unterminated cable will cause significant
distortion of the data edges. 

At 5MHz, the data "length" is around 30 metres and 10cm is somewhat
less significant.


Geoff

-- 
    Geoff Blake	              Please reply to:		Using Linux on  
    Chelmsford		geoff (a) palaemon . co . uk	Intel & NetBSD 
    Essex UK		  geoff @ g8gnz . ampr.org	 on Sun Sparc

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