Difference between IDE and SCSI ??

Tim ignored_mailbox at yahoo.com.au
Wed Feb 6 07:24:08 UTC 2008

On Tue, 2008-02-05 at 21:21 +0100, Nigel Henry wrote:
> Sorry for the delay in asking this, but what is the difference between using 
> 40 pin 40 wire, and 40 pin 80 wire IDE cables?

The high speed UDMA drives (66 MHz or faster) need the 80 wire cables,
slower speed drives do not.  As the other reply said, the extra 40 wires
are ground wires between signal wires, to minimise crosstalk between the
data lines.

> I ask because on one of my machines I have a fixed harddrive, and also a 
> harddrive caddy that uses one of the 5 ¼ slots, and is where I plug my 
> various drives in, that have various multiboot installs on them. With a 
> standard 80 wire ribbon cable it's impossible to connect both drives, so I 
> used the end connector for the fixed data drive, and used an extension cable 
> from the middle connector to reach the harddrive caddy. The only extension 
> cables I could find were 40 wire ones. I started to notice that there were 
> some bootup problems showing up, when booting some distros, so changed the 
> cables over, so that the 80 wire end connection was now connected to the 
> harddrive caddy that dealt with the OS's

I would not mess around with trying to extend the ribbon cabling.
You're really pressing your luck.  And it's not just read errors you
have to contend with, writing errors can be a disaster.

With the 80-wire cables, you need to use them the right way around.  The
end for the motherboard has to be connected to the motherboard.  That's
the end where the extra ground wires are terminated.  It does not matter
which drives go on which connectors at the other end, no matter what any
idiot says about it, so long as you don't leave an end-of-a-cable
connector not connected to anything.  Whether a slave or master goes at
the end or in the middle *only* matters when using cable-select.  Don't
mix and match one drive cable-select, and the other jumpered.  Do both
jumpered as master and slave, *OR* both jumpered as cable-select.

If you have troubles with cabling reaching far enough, buy a longer
lead.  There is a limit (457 mm or 18"), don't exceed it.  There's also
a minimum length (254 mm or 10").

If you have troubles with the number of drives you have, versus
connectors (e.g. some drives don't share a cable well), get another
interface card, so you've got more drive ports.

I'd recommend against using round IDE cables.  Their characteristics are
variable, and different from the standard.  And there's no way that they
can have the same characteristics as the high speed UDMA cables (signal
wire, grounded wire, repeated).

(This computer runs FC7, my others run FC4, FC5 & FC6, in case that's
 important to the thread.)

Don't send private replies to my address, the mailbox is ignored.
I read messages from the public lists.

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