Standard Disk Interconnect
| Industry | Computer hardware |
|---|---|
| Products | Standard Disk Interconnect (SDI), RAxx series disk drives (including RA90) |
The Standard Disk Interconnect (SDI, occasionally referred to as Standard Disk Interface) was created by Digital Equipment Corporation specifically for its RAxx series of disk drives, including models such as the RA90. This technology employed two bi-directional serial communication channels connecting the controller with the drive. One channel managed command messages (from controller to drive) and corresponding responses (from drive to controller), encompassing operations like seeking, parameter configuration, error reporting, and data transmission. The second channel carried continuous control signals (controller-to-drive) or status bits (drive-to-controller), typically around 6 bits in each direction, functioning similarly to dedicated signaling lines.
By transmitting control signals in a serial format, SDI enabled AC coupling through transformers, which helped eliminate issues related to ground loops and EMI over extended cable runs. This design supported connectivity across multiple building levels. Moreover, this serial interconnection was extended to include tape and solid state storage devices interfaced with storage subsystem controllers. While large VAX systems predominantly utilized the Hierarchical Storage Controller (HSC) family, SDI was also implemented in Unibus and BI Bus controllers. The first SDI-equipped controller released was the UDA50 designed for the Unibus.
Signals sent from the drive to the controller comprised:
- Index Pulse – signaling that the read/write head is positioned over sector 0 on a track.
- Sector Pulse – indicating other sector locations, used by the controller to monitor disk rotation.
- Read/Write Ready – confirming that the drive head has stabilized and is prepared for safe read or write actions.
Commands transmitted from the controller to the drive included Read Gate and Write Gate signals to engage the respective heads during data operations.
SDI accommodated a broad spectrum of self-timed bit rates, ensuring interoperability among various generations of disk drives and other storage technologies.
As an element of Digital's Storage Architecture (DSA), SDI integrated with the Mass Storage Control Protocol (MSCP), which aimed to unify communication protocols between operating system software and diverse storage hardware.
Further reading
- Chapter 7-The Storage Interconnect and Chapter 10-SDI Disk Drives.
- U.S. Patent 5,303,265 on controller-drive signalling electronics.
- U.S. Patent 4,825,406 on SDI protocol.