page_title: Hot swapping
section_title: Connectors
hierarchical_section_title: Hot swapping / Connectors
context_page_description: Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are said to be hot-swappable or hot-pluggable; likewise, components which do not are cold-swappable or cold-pluggable.
  Most computer hardware, such as CPUs and memory, are only cold-pluggable. However, it is common for high-end servers and mainframes to feature hot-swappable capability for other components, such as PCIe and SATA drives.
  A well-known example of hot swap functionality is the Universal Serial Bus connection, which allows users to add or remove peripherals such as a mouse, keyboard, printer, or portable hard drive. Such devices are characterized as hot-swappable or hot-pluggable depending on the supplier.
context_section_description: 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, 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.
  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, 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 at that speed.
  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.
caption: hot-swap connector corner pins