Quote:
Originally posted by MaggieL
... So instead of just drives, you can replace drive *controllers*. Or NIC cards. Telephony interfaces. Or whatever you have attached to the PCI bus that uses this technology.<p> It's not really *new* tech in terms of internet time either...this was a few years ago. [/b]
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Hot swap busses are old technology even for the military. One that I reviewed was a communication system for a new line of nuclear submarines. Any computer could and had to be removed or replaced while the entire communication system remained operational. It used an old (1970s) bus structure called something like IEEE 1324 (number long since forgotten) which is also used in aviation - and before fly by wire was yet common in commercial airplanes.
The most important problem in hot swapping are voltage differences - especially if the board has multiple voltages. It is critical to establish ground before making any other power or bus connection. If a board is powered by +12V, +5V, and -12V; but has no ground, then PN semiconductor junctions, normally used as isolators, can become conductors. IOW current flow in ICs where the IC is not designed to handle current flow - damage.
HP computers (pre-PCs), although not hot swappable, assumed that humans would make mistakes. Therefore ground was located on both ends of a board and ground pins were longer - to guarantee that ground would always be 'first to make' and 'last to break'.
USB is also a hot swappable bus. Current to a newly installed board must be current limited so that the board does not overdraw from the bus. Then software must recognize the new hardware, initialize the hardware, and inform all other systems (usually just the OS) that a new device is in existance. Another whole and different scenario is required for hot-swap removal. It requires coordination between software and hardware - something not always easily accomplished by the software side of engineering.
As a result of hot swapping and other features, the code for USB is about as complicated as the entire MS-DOS package. Linux had trouble implementing complex USB. The complexities of a hot swappable USB is why Transmeta is so important to Linux's future.
Hot swapping is not easily implemented since humans can vary the hot swapping process so easily. Code and hardware must be designed to withstand every 'innovative' hot swapping human.
Feature articles on hot swapping busses were in electronics magazines some years ago as newer, hot swap interface chips were developed.