Filename: CABLESEL.TXT
WWW URL: http://www.firmware.com/support/bios/cablesel.htm
FTP URL: ftp://ftp.firmware.com/text/cablesel.txt
Revision: 03/25/99 TLS Micro Firmware Technical Support
Summary: Description of Cable Select jumpering scheme including
issues with cables and host interfaces.
All newer IDE/EIDE hard drives can be jumpered as Cable Select (CS or CSEL). This is an alternate way to indicate which drive is master and which drive is slave (instead of jumpering one drive as master and one drive as slave). Cable Select jumpering requires a special IDE cable with pin 28 not connected to one of the drive connectors, which would configure the drive attached to that connector as the slave drive.
Cable Select jumpering is not widely used now, but may become more common as things move more towards Plug and Play, as this is part of the ATA PnP standard and Microsoft's PC97 standard. The idea is that drives can be installed easily without having to change jumpers on two drives anytime a drive is installed or removed. Cable Select is defined in the ATA-2 and ATA-3 specifications.
In order to use Cable Select jumpering, several conditions must be met. Both drives on a channel must support CSEL, both drives must be jumpered as CSEL, a CSEL cable must be used, and the host interface connector must support CSEL. For the host interface to support Cable Select, pin 28 must be grounded.
Although the Cable Select specification may simplify things in the future, there will probably be lots of confusion, especially on legacy systems, as this starts to be introduced. One problem will be in selecting the correct cable. Supposedly, the cables used for Cable Select will be clearly marked, with each connector labeled as Device 0 (or Master) or Device 1 (or Slave). If not clearly marked, it may not be easy to identify a CSEL cable visually. Pin 28 can be checked for continuity.
A Cable Select cable can be constructed in various ways. Pin 28 may be nonconnected to the connector at the end of the cable or to the connector in the middle of the cable. Another design would have the host interface connector in the middle and the two drives would plug into each end of the cable, with the connector at one of the ends not connected to pin 28.
If both drives are set for CSEL and the host interface supports CSEL, but a regular cable is used, both drives will be seen as master.
A Cable Select cable can be used with master/slave drive jumpering.
Another problem will be with host interfaces on legacy motherboards and controller cards. If pin 28 is not grounded on the host interface, drives connected to either connector on the CSEL cable will be seen as slave. It will be common to find that pin 28 is open or high on many older IDE interfaces. This can be checked with a voltmeter.
Here is a picture of the pin numbering on the IDE interface:
39 37 35 33 31 29 27 25 23 21 19 17 15 13 11 9 7 5 3 1
40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2