Serial general purpose input/output (SGPIO) is a four-signal (or four-wire) bus used between a host bus adapter (HBA) and a backplane. Of the four signals, three are driven by the HBA and one by the backplane. Typically, the HBA is a storage controller located inside a server, desktop, rack or workstation computer that interfaces with hard disk drives or solid state drives to store and retrieve data. It is considered an extension of the general-purpose input/output (GPIO) concept. The SGPIO specification is maintained by the Small Form Factor Committee in the SFF-8485 standard. The International Blinking Pattern Interpretation indicates how SGPIO signals are interpreted into blinking light-emitting diodes (LEDs) on disk arrays and storage back-planes. History SGPIO was developed as an engineering collaboration between American Megatrends Inc, at the time makers of back-planes, and LSI-Logic in 2004. SGPIO was later published by the SFF committee as specification SFF-8485. Host bus adapters The SGPIO signal consists of 4 electrical signals; it typically originates from a host bus adapter (HBA). iPass connectors (Usually SFF-8087 or SFF-8484) carry both SAS/SATA electrical connections between the HBA and the hard drives as well as the 4 SGPIO signals. Backplanes with SGPIO bus interface A backplane is a circuit board with connectors and power circuitry into which hard drives are attached; they can have multiple slots, each of which can be populated with a hard drive. Typically the back-plane is equipped with LEDs which by their color and activity, indicate the slot's status; typically, a slot's LED will emit a particular color or blink pattern to indicate its current status. SGPIO interpretation and LED blinking patterns Although many hardware vendors define their own proprietary LED blinking pattern, the common standard for SGPIO interpretation and LED blinking pattern can be found in the IBPI specification. On back-planes, vendors use typically 2 or 3 LEDs per slot – in both implementations a green LED indicates presence and/or activity – for back-planes with 2 LEDs per slot, the second LED indicates Status whereas in back-planes with 3 LEDs the second and third indicate Locate and Fail. Electrical characteristics of the SGPIO bus The SGPIO bus consists of 4 signal lines and originates at the HBA, referred to as the initiator and ends at a back-plane, referred to as the target. If a back-plane (or target) is not present the HBA may still drive the bus without any harm to