What is "AC" with old hard disk?

Hi,

I am transferring an old hard disk (20 Gbyte
Maxtor D740X) from an old PC to a newer one
(Dell Dimension 3000). On the old machine
the disk was the boot drive, and so was
configured as the Master device. On the newer
machine it will be a Slave device.

I have the "Jumper, CHS, and Install Guide"
for the drive, but there are two Slave
jumper settings. These are called "Slave"
and "Slave with AC", but nowhere does the
document explain was "AC" actually is.
(There is also "Master with AC" and "Cable
Select with AC".) What is "AC"?

I can't Google at the moment because my old
PC does not have the required encryption
protocols. This is why I am trying to
upgrade.
--
Regards,
Martin Leese
E-mail: LID
Web: http://members.tripod.com/martin_leese/