Standard Tape Interconnect

The Standard Tape Interconnect (usually referred to by the acronym, STI) is a DEC designed magnetic tape formatter to controller interface standard, intended to support sequential-type mass storage devices. It is a key element of the Digital Storage Architecture (DSA), used with PDP-11s and with VAX single and multiple CPU systems.

STI claims to offer high performance, availability, and configuration flexibility, and does so across a large range of configurations (ranging from small individual systems, to groups of large servers.

A variant, the Standard Disk Interconnect (SDI), is used with disk systems. It shares its physical connection technology (called the Standard Drive Bus, SDB) with STI; the higher layer protocols are adapted to random access storage needs. STI configurations are slightly more complex than SDI, as SDI controllers connect directly to disk drives, whereas in STI a formatter is interpolated between the STI controller and the magnetic tape drives attached to it.

Technical details

The SDB formatter 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.

Logically, at a higher layer, STI 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. This protocol is the Tape Mass Storage Control Protocol (TMSCP).

STI commands include:

• Start and stop the motion of the tape
• Space the tape forward or backward
• Rewind the tape
• Write an inter-record gap on the tape
• Select a configurable attribute (e.g. density)
• Ask a drive for its attributes and/or status
• Begin recovery from an error

STI Family of DSA Products

The family of products which implement the STI includes:

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

• STI drives
  • TA78
  • TA79
  • TA81
  • TA90

Further reading

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