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 the system; if one does exist, it can communicate back to the HBA using the 4th wire.
The SGPIO bus is an open collector bus with 2.0 kΩ pull-up resistors located at the HBA and the back-plane - as on any open collector bus information is transferred by devices on the bus pulling the lines to ground (GND) using an open collector transistor or open drain FET.
Signal lines of the SGPIO bus
SClock
The SGPIO bus has a dedicated clock line driven by the initiator (its maximum clock rate is 100 kHz), although many implementations use slower ones (typically 48 kHz).
SLoad
This line is synchronous to the clock and is used to indicate the start of a new frame of data; a new SGPIO frame is indicated by SLoad being high at a rising edge of a clock after having been low for at least 5 clock cycles. The following 4 falling clock edges after a start condition is used to carry a 4-bit value from the HBA to the back-plane; the definition of this value is proprietary and varies between system vendors.
SDataOut
This line carries 3 bits of data from the HBA to the backplane: the first bit typically carries activity; the second bit carries locate; and the third bit carries fail. A low value for the first bit indicates no activity and a high value indicates activity.
SDataIn
This line is used by the back-plane and indicates some condition on the back-plane back to the HBA. The first bit being high commonly indicates the presence of a drive. The two following bits are typically unused, and driven low. Because this line would be high for all 3 bits when no backplane is connected, an HBA can detect the presence of a back-plane by the second or third bit of the SDataIn being driven low.
The SDataIn and SdataOut then repeats with 3 clocks per drive until the last drive is reached, and the cycle starts over again.