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WD Scorpio ML320S-AF2
3.7
Hot Plug Support
SATA supports hot plugging (also known as "hot swapping"), the ability to swap out a failed hard drive
without having to power down the system or reboot. This capability contributes to both data
availability and serviceability without any associated downtime, making it a critical feature for
extending SATA into enterprise applications.
WD SATA drives support hot plugging only in systems where a SATA hard drive storage backplane is
used.
The Serial ATA revision 2.5 specification requires staggered pins for both the hard drive and drive
receptacles. Staggered pins mate the power signals in the appropriate sequences required for powering
up the hot plugged device. These pins are also specified to handle in excess of the maximum allowed
inrush current that occurs during drive insertion. SATA-compliant devices thus need no further
modification to be hot pluggable and provide the necessary building blocks for a robust hot plug
solution, which typically includes:
Device detection even with power downed receptacles (typical of server applications).
Pre-charging resistors to passively limit inrush current during drive insertion.
Hot plug controllers to actively limit inrush current during drive insertion.
3.7.1
Reliable hot plugging of disk drives requires careful design of specific issues. Generally, hot plugging is
only allowed when blind mating with a backplane, where there are controlled tolerances for alignment
of mating connectors. The main issue is preventing droop (a momentary drop in voltage) of the
backplane's voltage busses during insertion because of inrush current to charging bypass capacitors in
the disk drive.
3.7.1.1
There are two methods of limiting inrush current when hot plugging disk drives. They are hot plug
controller IC's and precharge resistors.
3.7.1.1.1 Hot Plug Controller IC's
Hot plug controller IC's ramp up voltages to the inserted disk drive at a controlled slow rate (a constant
dv/dt), after full connector mating has been established. This eliminates almost all inrush current. This
method does not require staggered precharge contacts, although there usually is a staggered ground
'mate first' pin(s) to ensure ground connection before making power and signal connections.
Sometimes added ground pins on both ends of the mating connector are staggered 'mate last' providing
the hot plug controller a clear indication of all other pins making solid contact. From an engineering
standpoint, this is the most reliable method of hot plugging disk drives, but is expensive to implement
on backplanes.
3.7.1.1.2 Precharge Resistors
Current limiting series precharge resistors on designated power pins make contact before hard power
connection is made, providing controlled partial charge of the disk drive's decoupling capacitors. This
partial charge is due to IR drop across the precharge resistor caused by disk drive DC loads at partial
power. A second current surge occurs when final power connections are made, completing charge of
the decoupling capacitors. Use of precharge resistors requires three stages of contact sequencing on the
mating connector. The first stage makes initial ground contact, establishing a ground reference between
the disk drive and backplane. It also discharges any ESD voltage between the two devices. The second
stage contacts connect the precharge resistors, supplying limited current to the inserting disk drive's
voltage busses. This allows partial charging of decoupling capacitors on the disk drive. Stage three
contacts make hard power and signals connections. The ideal value for precharge resistors is where peak
inrush current for both the precharge resistor connections and final hard power connections are similar
in amplitude. Even with precharge resistors, there may be some momentary droop of the backplane
2679-771320-A00
Hot Plug Technical Issues
Methods of Controlling Inrush Current
RELEASED 4/7/11 (WD CONFIDENTIAL)
Product Features
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