Editing Hot swapping (section)

==Mechanical and electrical design considerations==
Machines that support hot swapping need to be able to modify their operation for the changed [[computer configuration|configuration]], either automatically on detecting the change, or by user intervention. All electrical and mechanical connections associated with hot-swapping must be designed so that neither the equipment nor the user can be harmed while hot-swapping. Other components in the system must be designed so that the removal of a hot-swappable component does not interrupt operation.

=== Protection against electrostatic damage ===
Protective covering plates, shields, or bezels may be used on either the removable components or the main device itself to prevent operator contact with live powered circuitry, to provide antistatic protection for components being added or removed, or to prevent the removable components from accidentally touching and shorting out the powered components in the operating device.

Additional guide slots, pins, notches, or holes may be used to aid in proper insertion of a component between other live components, while mechanical engagement latches, handles, or levers may be used to assist in proper insertion and removal of devices that either require large amounts of force to connect or disconnect, or to assist in the proper mating and holding together of power and communications connectors.

=== Component shutdown procedure before unplugging ===
Some implementations require a component shutdown procedure prior to removal. This usually results in a simpler design, but such devices are not robust in the case of component failure. In such cases, if a component is removed while it is being used, the operations to that device fail and the user is responsible for retrying if necessary. In practice, this can be an advantageous trade-off for certain designs where cost matters more than reliability.

More complex implementations may recommend but do not require that the component be shut down. In the suboptimal case a component is removed without being shut down, these implementations usually have sufficient [[redundancy (engineering)|redundancy]] to allow essential operation to continue. In these systems hot swap is normally used for regular maintenance to the computer, or to replace a broken component.

=== Connectors ===
[[Image:SPARCstation20 scsi cradle with drive.jpg|thumb|Sun SPARCstation hot swappable [[Single Connector Attachment]] (SCA) drive cradle{{cn|date=February 2017}}<!--See talk page: https://en.wikipedia.org/w/index.php?title=Talk%3AHot_swapping&type=revision&diff=763222423&oldid=760279983 -->]]
Most modern hot-swap methods use a specialized connector with staggered pins, so that certain pins are certain to be connected before others. Most staggered-pin designs have ground pins longer than the others, ensuring that no sensitive circuitry is connected before there is a reliable system ground. The other pins may all be the same length, but in some cases three pin lengths are used so that the incoming device is grounded first, data lines connected second, and power applied third, in rapid succession as the device is inserted. Pins of the same nominal length do not necessarily make contact at exactly the same time due to mechanical tolerances, and angling of the connector when inserted.

At one time staggered pins were thought to be an expensive solution,{{Citation needed|date=January 2013}} but many contemporary connector families now come with staggered pins as standard; for example, they are used on all modern serial SCSI disk-drives.  Specialized hot-plug power connector pins are now commercially available with repeatable DC current interruption ratings of up to 16&nbsp;A.  [[Printed circuit board]]s are made with staggered edge-fingers for direct hot-plugging into a backplane connector.

Although the speed of plugging cannot be controlled precisely, practical considerations will provide limits that can be used to determine worst-case conditions.  For a typical staggered pin design where the length difference is 0.5&nbsp;mm, the elapsed time between long and short pin contact is between 25&nbsp;ms and 250&nbsp;ms.  It is quite practical to design hot-swap circuits that can operate at that speed.

[[Image:Connector corners.svg|thumb|right|350px|Hot-swap connector corner pins]]
As long as the hot-swap connector is sufficiently rigid, one of the four corner pins will always be the first to engage.  For a typical two-row connector arrangement this provides four first-to-make corner pins that are usually used for grounds.  Other pins near the corners can be used for functions that would also benefit from this effect, for example sensing when the connector is fully seated.  This diagram illustrates good practice where the grounds are in the corners and the power pins are near the center.  Two sense pins are located in opposite corners so that fully seated detection is confirmed only when both of them are in contact with the slot.  The remaining pins are used for all the other data signals.

=== Power electronics ===
The DC power supplies to a hot-swap component are usually [[pre-charge]]d by dedicated long pins that make contact before the main power pins.  These pre-charge pins are protected by a circuit that limits the inrush current to an acceptable value that cannot damage the pins nor disturb the supply voltage to adjacent slots.  The pre-charge circuit might be a simple series [[resistor]], a [[negative temperature coefficient]] (NTC) resistor, or a [[current-limiter]] circuit.  Further protection can be provided by a "soft-start" circuit that provides a managed ramp-up of the internal DC supply voltages within the component.

A typical sequence for a hot-swap component being plugged into a slot could be as follows:
#Long ground pins make contact; basic electrical safety and ESD protection becomes available.
#Long (or medium) pre-charge pins make contact; decoupling capacitors start to charge up.
#Real time delay of tens of milliseconds.
#Short power/signal pins make contact.
#Connector becomes fully seated; power-on reset signal asserted within component
#Soft-start circuit starts to apply power to the component.
#Real time delay of tens of milliseconds.
#Soft-start circuit completes sequence; power-on reset circuit deasserted
#Component begins normal operation.

Hot-swap power circuits can now be purchased commercially in specially designed [[Application-specific integrated circuit|ASICs]] called hot-swap power managers (HSPMs).

=== Signal electronics ===
Circuitry attached to signal pins in a hot-swap component should include some protection against [[electrostatic discharge]] (ESD). This usually takes the form of clamp diodes to ground and to the DC power supply voltage. ESD effects can be reduced by careful design of the mechanical package around the hot-swap component, perhaps by coating it with a thin film of conductive material.

Particular care must be taken when designing systems with bussed signals which are wired to more than one hot-swap component.  When a hot-swap component is inserted its input and output signal pins will represent a temporary short-circuit to ground.  This can cause unwanted ground-level pulses on the signals which can disturb the operation of other hot-swap components in the system.  This was a problem for early [[parallel SCSI]] disk-drives.  One common design solution is to protect bussed signal pins with series diodes or resistors. CMOS buffer devices are now available with specialized inputs and outputs that minimize disturbance of bussed signals during the hot-swap operation.  If all else fails, another solution is to [[quiesce]] the operation of all components during the hot-swap operation.