[ILUG] hard disk

[Dunphy Richard-rdunph01]

> -----Original Message-----
> From: Dermot Gorman [mailto:dgm at physics.dcu.ie]
> Sent: 08 March 2000 12:31
> To: ilug at linux.ie
> Subject: [ILUG] hard disk
> 
> 
> Hi,
> way off topic but - I'm buyin a new EIDE hard disk, could anybody
> tell me what the difference between UDMA33 and UDMA66 is.
> Will it work on an older motherboard, i.e. of pentium 166 vintage?
> Dermot.
> 

UDMA is an extenson of ATA PIO Mode 4. For UDMA33 the data is clocked on
both the falling and rising edges. UDMA66 requires changes to the setup and
hold times of the ATA interface. These changes make it more susceptable to
noice which is why to use UDMA66 you need to get the 80 connector IDE cables
which have more ground connections. The upshot of all this was for PIO Mode
4 you had a theoretical burst rate of 16Mb/s which was increased to 33Mb/s
with UDMA33 and now 66Mb/s with UDMA66.

All things going well the drive should default back to whatever your BIOS
supports. But there are some exceptions. Most UDMA66 drives on older BIOSes
(even Some UDMA33 BIOSes) need you to run a little utility off the
manufacturers website which essentially forces the drive out of UDMA66 mode
and to PIO4/UDMA33 mode.

There is little performance gain to be gotten from UDMA66 over UDMA33 unless
you plan to do some large file transfers. The drives still have very similar
access times of 8 to 9ms. And to get the full UDMA66 performance you need to
invest in the new IDE cables.

But! I still would buy the UDMA66 over the UDMA33. The reason being that
most UDMA33 drives have typically 512Kb of cache on the drive, while most
UDMA66 drives have 2Mb. This *does* make a huge difference.

Hope that is of help
RikD.