Standard Drive Bus

The Standard Drive Bus (sometimes referred to by the acronym, SDB) is the lowest, hardware level of several DEC designed mass storage drive to controller interfaces. It is a key element of the Digital Storage Architecture (DSA), used with PDP-11s and with VAX single and multiple CPU systems.

A new standard, it was introduced (in 1981) because no existing standard had the capabilities that DEC deemed necessary in the mass storage arena. It is part of the Standard Disk Interconnect and Standard Tape Interconnect; both add higher layer protocols which are adapted to the needs of their supported device types.

The SDB drive to controller interconnect is a serial link which connects each drive directly (i.e. individually), or 'radially' (a term for such a device connection approach which DEC also used in its early machines), to its controller.

Electrically, the SDB consists of four signal carriers, using small-diameter coaxial cables; 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 SDB signals are transformer coupled (economically feasible with such a small number of links), so the drive and controller do not require a common DC ground, simplifying installation. Transformer coupling also eliminates unpredictable and unrepeatable "ground loop" problems, and provides for installation flexibility, since SDB drives can be physically added to or removed from a subsystem without disrupting its operation.

These four cables are emplaced in a single connector for actual installation of SDB units.

Further reading

• Digital Storage Technology Handbook, 1989 (EC-H0374-45/89) - Chapter 4, The Digital Storage Interconnects
• Digital Large System Mass Storage Handbook, 1986 - Chapter 7, The Storage Interconnect