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'SGPIO' is an acronym for 'Serial General Purpose Input/Output' which is a 4-signal (or 4-wire) bus used between a Host Bus Adapter (HBA) and a backplane. Out of the 4 signals, 3 are driven by the HBA and 1 is driven 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 (HDDs) to store and retrieve data.
A backplane is a circuit board with connectors and power circuitry where hard drives can plug in. The backplane can have multiple slots, where each slot can be populated with a hard drive. Typically the backplane if outfitted with LEDs that indicate the status of that slot. The LEDs for each slot and their color indicate a particular state of that slot. Also an LED many emit a particular blink pattern to indicate a particular status for that slot.
There is no public standard that defines the exact LED color and blinking pattern for the LEDs, although it is commonly accepted that a green LED illuminated will indicate hard drive activity. A solid red LED commonly indicates that the drive in that slot has a hardware failure, and a blinking red LED indicated that data is being restored to that drive following a failure and physical replacement of the drive. This is referred to as the drive being rebuilt.
Another common feature is 'locate', where a user through hard drive, HBA or RAID management software can opt to locate a drive. This is often indicated by a solid third color LED (such as blue) or a rapidly simultaneous blinking of both LEDs on backplane with only 2 LEDs per slot.
The SGPIO bus consists of 4 signals lines and originates at the HBA, referred to as the 'initiator'. The SGPIO-bus ends on a backplane referred to as the target. If a backplane does not exist, the HBA may still drive the bus without any harm caused to the system. If a backplane (or target) does exist, it can communicate back to the HBA using the 4th wire.
The SGPIO bus is an open collector bus with 2.0kΏ pull-up resistors located at the HBA and the backplane. 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.
'SClock'
The SGPIO bus has a dedicated clock line, driven by the initiator. The maximum clock rate is 100kHz, although many implementations use clock rates that are slower (typ 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 frame in SGPIO is indicated by the 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 backplane. The definition of this value is proprietary and is defined between system vendors.
'SDataOut'
This line carries 3 bits of data from the HBA to the backplane, where the first bit typically carries 'activity', second bit carries 'locate' and the third bit carries 'fail'. A low value for the first bit indicates 'not activity' or inactive, and high indicates activity.
'SDataIn'
This line is used by the backplane and indicates some condition on the backplane 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 high, a HBA can detect the presence of a backplane 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.
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 a cables are removed and re-inserted. Other vendors send data only when there is a need to update the LED pattern.
The SGPIO spec is given the official name SFF-8485 and is maintained by the SFF Committee.
A collection of SFF-specifications is found on: [ftp://ftp.seagate.com/sff SFF Documents]
This spec in particular is located at: [ftp://ftp.seagate.com/sff/SFF-8485.PDF SFF-8485]
SGPIO and the SGPIO spec is generally adopted and implemented in products from most major HBA and Storage Controller vendors such as LSI, Intel, Adaptec, NVIDIA, Broadcom and PMC-Sierra.
Most products shipping with support for SAS and SATA drives support this standard.
The SGPIO spec calls for the target to turn off all indicators when SClock, SLoad and SDataOut has been high for 64ms. In practice this is not consistently followed by all vendors. Also, in implementation by some vendors the clock may be halted sporadically or stopped during or between cycles. Another impractical variation between vendors is how the clock is left after a cycle.
The idea behind this specification was to be able to use low cost CPLDs or microcontrollers on a backplane to drive LEDs. In practice, it has been found that there are variations in timing and interpretations of the bits between vendors, thus a simple CPLD would only work for a specific implementation thoroughly tested with one product from one vendor. A microcontroller is more applicable for this purpose, although the 4-bit SGPIO interface is a custom bus not found on micros. Also, sampling of the 4-bit lines using and implemented in a series of backplane controller chips named the MG9071, MG9072 and MG9073S.
These chips will receive 1 or 2 SGPIO streams and drive LEDs accordingly. The latest chip from AMI, the MG9073S, can be configured by pull-up and pull-down resistors to adopt to 16 different configurations of SGPIO busses and drive LEDs accordingly. Since the availability of these chips from AMI, major OEMs like NEC, Hitachi, SuperMicro, IBM, Sun Microsystems, and others are using these chips on their backplanes to receive the SGPIO streams from a variety of HBA vendors and on-board controller chips, and consistently drive LEDs with a pre-determined blinking pattern.
American Megatrends on Wikipedia
American Megatrends, Inc. Company Website AMI
Backplane Controllers from AMI
Backplanes with SGPIO bus interface
A backplane is a circuit board with connectors and power circuitry where hard drives can plug in. The backplane can have multiple slots, where each slot can be populated with a hard drive. Typically the backplane if outfitted with LEDs that indicate the status of that slot. The LEDs for each slot and their color indicate a particular state of that slot. Also an LED many emit a particular blink pattern to indicate a particular status for that slot.
There is no public standard that defines the exact LED color and blinking pattern for the LEDs, although it is commonly accepted that a green LED illuminated will indicate hard drive activity. A solid red LED commonly indicates that the drive in that slot has a hardware failure, and a blinking red LED indicated that data is being restored to that drive following a failure and physical replacement of the drive. This is referred to as the drive being rebuilt.
Another common feature is 'locate', where a user through hard drive, HBA or RAID management software can opt to locate a drive. This is often indicated by a solid third color LED (such as blue) or a rapidly simultaneous blinking of both LEDs on backplane with only 2 LEDs per slot.
Electrical Characteristics of the SGPIO bus
The SGPIO bus consists of 4 signals lines and originates at the HBA, referred to as the 'initiator'. The SGPIO-bus ends on a backplane referred to as the target. If a backplane does not exist, the HBA may still drive the bus without any harm caused to the system. If a backplane (or target) does exist, it can communicate back to the HBA using the 4th wire.
The SGPIO bus is an open collector bus with 2.0kΏ pull-up resistors located at the HBA and the backplane. 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. The maximum clock rate is 100kHz, although many implementations use clock rates that are slower (typ 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 frame in SGPIO is indicated by the 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 backplane. The definition of this value is proprietary and is defined between system vendors.
'SDataOut'
This line carries 3 bits of data from the HBA to the backplane, where the first bit typically carries 'activity', second bit carries 'locate' and the third bit carries 'fail'. A low value for the first bit indicates 'not activity' or inactive, and high indicates activity.
'SDataIn'
This line is used by the backplane and indicates some condition on the backplane 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 high, a HBA can detect the presence of a backplane 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 a cables are removed and re-inserted. Other vendors send data only when there is a need to update the LED pattern.
The SGPIO Specification
The SGPIO spec is given the official name SFF-8485 and is maintained by the SFF Committee.
A collection of SFF-specifications is found on: [ftp://ftp.seagate.com/sff SFF Documents]
This spec in particular is located at: [ftp://ftp.seagate.com/sff/SFF-8485.PDF SFF-8485]
Adoption of the SGPIO Specification
SGPIO and the SGPIO spec is generally adopted and implemented in products from most major HBA and Storage Controller vendors such as LSI, Intel, Adaptec, NVIDIA, Broadcom and PMC-Sierra.
Most products shipping with support for SAS and SATA drives support this standard.
SGPIO Timeout Conditions
The SGPIO spec calls for the target to turn off all indicators when SClock, SLoad and SDataOut has been high for 64ms. In practice this is not consistently followed by all vendors. Also, in implementation by some vendors the clock may be halted sporadically or stopped during or between cycles. Another impractical variation between vendors is how the clock is left after a cycle.
Backplane Implementations of the SGPIO bus
The idea behind this specification was to be able to use low cost CPLDs or microcontrollers on a backplane to drive LEDs. In practice, it has been found that there are variations in timing and interpretations of the bits between vendors, thus a simple CPLD would only work for a specific implementation thoroughly tested with one product from one vendor. A microcontroller is more applicable for this purpose, although the 4-bit SGPIO interface is a custom bus not found on micros. Also, sampling of the 4-bit lines using and implemented in a series of backplane controller chips named the MG9071, MG9072 and MG9073S.
These chips will receive 1 or 2 SGPIO streams and drive LEDs accordingly. The latest chip from AMI, the MG9073S, can be configured by pull-up and pull-down resistors to adopt to 16 different configurations of SGPIO busses and drive LEDs accordingly. Since the availability of these chips from AMI, major OEMs like NEC, Hitachi, SuperMicro, IBM, Sun Microsystems, and others are using these chips on their backplanes to receive the SGPIO streams from a variety of HBA vendors and on-board controller chips, and consistently drive LEDs with a pre-determined blinking pattern.
More Information on Backplanes and Backplane Controllers
American Megatrends on Wikipedia
American Megatrends, Inc. Company Website AMI
Backplane Controllers from AMI
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