Western Digital caviar blue xl500s Technical Reference Manual page 31

XL500S Product Features
previous 0.44 millisecond calculated minimum staggered pin delay allowed. The SATA
specification also states that the average effective input capacitance of most of the disk drives
tested was 20 mF, the same value used in the previous calculations. The hot plug verification
test procedure later in this document includes a test to verify this information.
3.11.1.5 SATA Disk Drive Hot Plug Design Issues
The SATA specification provides pins for series precharge resistors allowing use of this less
expensive implementation. This does not exclude the backplane designer from using hot plug
controller IC's for a more robust design. The only design requirements for the SATA disk drive
its self are providing staggered precharge contacts on the hot plug connector per the SATA
specification and ensuring that decoupling capacitors can tolerate a momentary inrush current
of about 1 ampere.
3.11.1.5.1 Future Disk Drive Design Change Issues
Any increase in effective decoupling capacitance above 20 mF on any of the disk drive's
voltage busses could affect the hot plug compatibility. Any significant change in power-up DC
load at power up could also be a problem with backward compatibility. Both these issues
need new specification items in drive specifications to set limits for both the disk drive
designers and designers of hot plug enclosures.
3.11.1.6 Hot Plug Removal Issues
Generally, removal of hot plug disk drives does not present any technical issues. Drives are
usually in a low power state prior to removal. Decoupling capacitors in the disk drive will
continue to provide power long enough to soften the sudden loss of current. Bulk capacitors
in the power supply and backplane will absorb the drop in power long enough for the
regulators to adjust.
3.11.1.7 ESD Issues
Prior to insertion, disk drives can have static charges of thousands of volts relative to the
enclosure. This charge needs discharging during the insertion process. If insertion guides are
ESD resistive, they will discharge the static charge prior to contact with the mating connector.
If the guides are not conductive, then any static charge will discharge at first contact of the
mating connector ground pins. This sudden discharge will normally not cause any problems
because the static charge of the disk drive is common mode to all internal circuitry making
internal differential current and voltages low during discharge. Because of the unpredictable
nature of ESD, use of ESD resistive (not metal) insertion guides is a safer solution since it
gradually discharges any static charge. Having the disk drive enclosed in a canister also helps
ensure proper discharge. It is the responsibility of the hot plug enclosure designer to handle
ESD issues in a safe manner.
3.11.1.8 Drive Canisters
Hot pluggable disk drives can be enclosed in a canister for added protection. This added
protection is both mechanical and electrical. The main electrical protection is from ESD.
Canisters prevent any local discharge onto sensitive parts of the disk drive during handling
and insertion. Canisters do increase the cost of the system.
2679-701211-A06 RELEASED 10/21/10 (WD CONFIDENTIAL) 23