The Standard Disk Interconnect (SDI, sometimes Standard Disk Interface) was used by

Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...

for its line of RAxx disks, for example the

RA90 The RA90 disk drive was the first Digital Equipment Corporation drive to be based on "thin film" technology. Prior to the RA90 all Digital disk drives used "oxide" disks, which were an aluminum disk coated with a polyurethane binder resin containi ...

. There were two bi-directional

serial communication In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are ...

s paths between the

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and the

drive Drive or The Drive may refer to: Motoring * Driving, the act of controlling a vehicle * Road trip, a journey on roads Roadways Roadways called "drives" may include: * Driveway, a private road for local access to structures, abbreviated "drive" ...

. One path was for messages (from the controller to the drive) and responses (from the drive to the controller); messages controlled seeking, setting drive parameters, reporting errors, etc. This path was also used to transmit

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to and from the drive. The other path consisted of a continuously transmitted set of control (controller-to-drive) or status (drive-to-controller) bits. There were about 6 bits in each direction, and they served the same purpose as having six dedicated lines in each direction. By encoding control signals serially they could be

AC-coupled Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental ...

(with a transformer) to prevent ground loop problems and

EMI EMI Group Limited (originally an initialism for Electric and Musical Industries, also referred to as EMI Records Ltd. or simply EMI) was a British transnational conglomerate founded in March 1931 in London. At the time of its break-up in 201 ...

issues between the controller and the drives, which could be connected by long cables, even to different floors in a building. The same serial interconnect was used for

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and solid state storage devices to the storage subsystem controllers. Large

VAX VAX (an acronym for Virtual Address eXtension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The VA ...

systems primarily used the Hierarchical Storage Controller (HSC) series but Unibus and BI Bus controllers to SDI were built as well. The UDA50 for the Unibus was the first controller shipped with the SDI interface. Among the signals the drive sent to the controller were: *Index Pulse - the

drive head A disk read-and-write head is the small part of a disk drive which moves above the disk platter and transforms the platter's magnetic field into electrical current (reads the disk) or, vice versa, transforms electrical current into magnetic fi ...

is over the beginning of

sector Sector may refer to: Places * Sector, West Virginia, U.S. Geometry * Circular sector, the portion of a disc enclosed by two radii and a circular arc * Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc * Spherical sector, a po ...

0 of the track. *Sector Pulse - the drive head is over the beginning of some other sector. The controller counted these pulses to know where the drive was, rotationally. *Read/Write Ready - the drive head has settled over some track, and it is safe to read or write. The controller signals to the drive included signals such as Read Gate (turn the Read Head on) and Write Gate (turn the Write Head on). The data transfer could be over a wide range of bit rates as the system was self clocked. This enabled multiple generations of disk technology to use the SDI, as well as other storage devices. The SDI was an element of the Digital Storage Architecture (DSA) which included the

Mass Storage Control Protocol The Mass Storage Control Protocol (MSCP) is a protocol that was designed by Digital Equipment Corporation of Maynard, Massachusetts for the purposes of controlling their high-end mass storage options. First implemented in the HSC50 hierarchical ...

(MSCP), which specified communications means between OS software and all flavors of storage devices.

Further reading