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What is the hot swap circuit design?

Hot-swap circuit design is very widely used, and it is a protection measure for components and chips of hot-swappable devices. Usually, the hot plug is used to isolate and buffer the signal, and is processed by devices such as 244 and 245. And the current-limiting resistor and 0.1uF filter capacitor are added to the input signal, and the output signal is usually output directly from 244, 245. Also, in addition to over-buffer isolation, for signals such as PCI interfaces, it is usually necessary to control the power-on, which is the hot-swap technology of the PCI bus.
Ordinary hard disk hot swap
Previous hard disk heads did not have the function of automatic docking. When the power was on, the head was “flying” on the platter. Before the system was powered off, a dedicated command called “Park” was used to return the head. Otherwise, it is possible that the disk is not able to return to the position due to the momentary stop of the disk, causing the disk to be “scraped” by the magnetic head.
The hard disk only flies on the surface of the disc when the data is read. At the end of a reading action, the head is automatically homed and docked immediately. At the same time, the hard disk has the function of delaying power off. That is, when the power supply of the system is suddenly lost, the controller of the hard disk itself can automatically detect this change, and then force the head to stop the execution of the current read and write command and normalize the head. This design greatly enhances the safety factor of the hard disk in case of unexpected power failure. Therefore, the possibility of disc damage is actually extremely low. But this does not mean that hot swapping the hard drive is not dangerous. Because the hard disk is plugged and unplugged in the power-on state, it will generate a transient inrush current. In the past, we thought that this was the main culprit for causing the hard disk to be plugged and unplugged. However, in fact, the hard disk power interface circuit has a relatively large tolerance to such transient current changes, and most of the time does not cause the hard disk circuit board to be burned. The real danger comes from the hard drive’s data line! When the hard disk data cable is plugged and unplugged under power, abnormal current and voltage drop will occur on the data line, causing multiple precision control chips to be burned. This is the real “hard disk killer”.
Therefore, as long as we can ensure that the order of plugging and unplugging the power cable and data cable is correct, that is, when plugging in the hard disk, first connect the data cable, then connect the power cable; when “pull” the hard disk, the opposite is true, first pull the power cable, then pull out Data line. In this way, hard disk hot swap is not a fantasy!

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