Standard Disk Interconnect

The Standard Disk Interconnect (usually referred to by the acronym, SDI; sometimes called the Standard Disk Interface) is a DEC designed disk drive to controller interface offering high performance, availability, and configuration flexibility. These characteristics make it suitable for use with PDP-11s and with VAX single and multiple CPU systems.

SDI Family of DSA Products

The family of DSA products which implement the SDI consists of:

• SDI Disk controllers:
  • KDA50 (QBUS)
  • KDB50 (VAXBI)
  • KDM70 (XMI)
  • UDA50 (UNIBUS)

• SDI Hierarchical Storage Controllers (HSCs):
  • HSC40
  • HSC50
  • HSC60
  • HSC65
  • HSC70
  • HSC90
  • HSC95

• SDI Disk Drives:
  • RA60
  • RA70
  • RA71
  • RA72
  • RA73
  • RA80
  • RA81
  • RA82
  • RA90
  • RA92

• SDI Solid State Disk Drives
  • ESE 20
  • ESE 25
  • ESE 50

SDI Features

To support DSA data integrity goals and provide compatibility and interchange, SDI controllers include advanced data integrity features:

• An advanced Error Correction Code (ECC) more than seven times as powerful as the industry's most common code.
• An Error Detection Code (EDC) which verifies ECC computation logic and the ECC correction process, checks controller data paths, and indicates a forced error in case a hard media error causes sector replacement.
• A Bad Block Replacement Algorithm which dynamically substitutes good blocks for potentially defective ones. The goal of the DSA is to anticipate developing defects and retire blocks containing them before user data loss occurs.
• Automatic Revectoring for transparent access to replaced blocks.
• Quadruplicate Headers for verification of read/write head position prior to data transfer, even if media defects exist in the header area.
• Firewalling against single failure points to detect and isolate single component failures so that no erroneous data is passed to the user.

SDI Interconnect

The SDI drive to controller interconnect is a serial bus which connects each drive directly, or radially, to its controller.

Electrically, SDI consists of four coaxial signal carriers, two for moving user data, commands, and responses between drive and controller, and two for continuous real time transmission of drive and controller status information. The SDI is transformer coupled, so drive and controller do not require a common DC ground. Transformer coupling both eliminates unpredictable and unrepeatable "ground loop" problems, and provides for installation flexibility, since SDI drives can be physically added to or removed from a subsystem without disrupting its operation.

Logically, SDI consists of a protocol by which a controller can recognize a drive's characteristics, direct its mechanical operation, store data on or retrieve data from it, and recognize and recover from error conditions.

Further reading

• Digital Storage Technology Handbook, 1989 (EC-H0374-45/89) - Chapter 4, The Digital Storage Interconnects

External links

• SDI Disk Controllers - brochure