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. SGPIO implementation There are varieties in how the SGPIO bus is implemented between vendors of HBAs and storage controllers - some vendors will send a continuous stream of data which is advantageous to quickly update the LEDs on a backplane after cables are removed and re-inserted, while others send data only when there is a need to update the LED pattern. Adoption of the SGPIO specification SGPIO and