Cable Select vs. Master/slave settings

[Gene Heskett]

On Sunday 07 August 2005 15:53, Tony Nelson wrote:
>At 3:19 PM -0400 8/7/05, Gene Heskett wrote:
>>Nice idea Claude, but can you tell us how to tell the difference
>>between the cables so that we can properly identify them?
>
>Just google on "cable select" (with the quotes) and you'll find that
> a Cable Select cable has pin 28 connected to a ground wire at one
> device connector (master) and not connected at the other (slave);
> normally this is done by punching out a little bit of wire 28 (on a
> 40 wire cable) just past the middle connector, so you'll see a
> little hole in the cable.  You'd also find that, as normally a
> single device should be at the end of the cable (slave) and a
> single device should be master, so using a Cable Select cable with
> only one device is, umm, problematical.
>
>If one must use a Cable Select cable for only one device and it
> doesn't seem to work quite right at either position, I suppose one
> could just cut the cable after the middle connector, which would
> make it the end connector.  Cables are cheap.

That solution has much to recommend it in terms of signalling 
integrity.  As a general rule, there should <b>never</b> be any cable 
beyond the last connected device, and this is particularly dangerous 
in a scsi environment where as little as 3" of unterminated cable can 
and will cause data error problems from the open end echo's.  I've 
been looking at udma100/133 cabling for years trying to figure out 
just how the hell they get away with the sloppiness and still make it 
work.  But that sloppiness in termination also restricts the maximum 
length (18" absolute max) to something thats not always reachable in 
a big tower case.

-- 
Cheers, Gene
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