Hot swapping and hot plugging are terms used to separately describe the functions of replacing system components; hot swapping describes changing components like fans and power supplies which do not interact with the system software, while hot plugging describes changing or adding components like hard drives which do interact with the operating system. Both terms describe the ability to remove and replace components of a machine, usually a computer, while it is operating. For hot swapping once the appropriate software is installed on the computer, a user can plug and unplug the component without rebooting. A well-known example of this functionality is the Universal Serial Bus (USB) that allows users to add or remove peripheral components such as a mouse, keyboard, or printer. It usually requires more sophisticated software and hardware than does plug-and-play.
Hot swapping may be used to add or remove peripherals or components, to allow a device to synchronize data with a computer, and to replace faulty modules without interrupting equipment operation.
Equipment may be designed with redundancy so that in the event of the failure of a component, other parts of the system carry out its functions while the faulty component is removed and a replacement connected. For example, computer RAID disk arrays allow a faulty disk to be hot-swapped for a new one; the new one is configured to become part of the array automatically or by user command. A machine may have dual power supplies, each adequate to power the machine; a faulty one may be hot-swapped.
Some implementations require a component shutdown procedure prior to removal. This simplifies the design, but such devices are not robust in the case of component failure. 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, although this is not usually considered to be a problem.
More complex implementations may recommend but not require that the component be shut down, with sufficient redundancy in the system to allow operation to continue if a component is removed without being shut down. In these systems hot swap is normally used for regular maintenance to the computer, or to replace a broken component.
There are two slightly differing meanings of the term hot swapping.
Most modern hot-swap methods use a specialized connector with staggered pins, so that certain pins are certain to be connected before others. At one time staggered pins were thought to be an expensive solution, 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 A. Printed circuit boards are made with staggered edge-fingers for direct hot-plugging into a backplane connector.
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; in some cases three pin lengths are used.
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 mm (0.020 inches), the elapsed time between long and short pin contact is between 25 ms and 250 ms. It is quite practical to design hot-swap circuits that can operate over that dynamic range. 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.
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.
The DC power supplies to a hot-swap component are usually pre-charged 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:
Hot-swap power circuits can now be purchased commercially in specially designed ASICs called Hot Swap Power Managers (HSPMs).
In the mid 1990s several radio transmitter manufactures in the US started offering swappable high power RF transistor modules.
The reintroduction of power modules has been good for the radio transmitter industry, as it has fostered innovation. Modular transmitters have proven to be more reliable than tube transmitters, when the transmitter is properly chosen for the conditions at the transmitting site.
Companies that produce transmitters using power modules
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.