Why some computer peripherals should not be disconnected without turning off things?

As far as I know, some cables should not be disconnected from computer while it and/or the device is running and persistent hardware damage can occur if they are unplugged errorneously.

Currently the table is like this:

• USB, Firewire, Ethernet, HDMI - can connect and disconnect without caution;
• VGA Monitor - OK;
• Serial port (COM) - OK probably;
• SATA - should be more or less OK;
• IDE, PS/2 - not recommended;
• Parallel port (LPT) - very not recommended;
• CPU, RAM, PCI, motherboard to PSU slot, ... - I don't know.

I tried or seen hot disconnections for all mentioned above (expect of LPT and the last point) without any problems.

Why the slow or device can be damaged by sudden disconnection? What happens when some (not all) pins are not connected?

Does it still apply for ( LPT or PS/2 or IDE ) USB adapters? Can they also be damaged or "unplugged cable => burned port" is only for old legacy hardware?

Can a cable with damaged wires (but without any short circuits) damage the port or device?

Note: just software issues (data loss, operating system crash) is out of scope in this question. It's only about hardware failures that cannot be fixed without bying things.

• Actually hot-swapping is not OK for VGA and COM and although many devices are designed to survive hot-swapping, but it's not a mandatory feature. – AndrejaKo Dec 8 '12 at 20:29
• The worst I have had with VGA hot-swapping, together with broken video drivers, were some messed-up resolutions when one reconnected the display. But then, YMMV and some equipment might be badly designed. – Renan Dec 8 '12 at 20:41
• I've never seen hot-swapping an LPT, COM, or PS/2 port do any damage or fail in any way in over two decades. – user207421 Dec 8 '12 at 23:16
• COM and LPT are MS-DOS language. Outside of Wintel, they are called serial and parallel. Serial ports that do not hot plug are garbage. An important use case for serial ports is to serve as a more or less hidden instrumentation interface to equipment where you can plug in a terminal at any time. – Kaz Dec 9 '12 at 0:32

I think it is threefold:

1. When power supply of various IC's is not connected before signals on other pins, the semiconductor can act as a thyristor between power supply rails, which results in short circuit the PSU (failing the whole PC) and high power dissipation or blowing the chip(s on the device).

2. The computer for many devices just isn't aware of them being plugged or unplugged. The operating system will not be able to use it as it is unaware of its existence OR the operating system is trying to use a device that is no longer available. Only few buses are able to cope with hot plugging (eg. USB).

3. (inductive) power surges due to unplugging can easily blow poorly designed hardware. I blew out the DAC's on my video processor once...

• Can "2." permanently damage the hardware? – Vi0 Dec 8 '12 at 20:30
• Actually most newer connection methods work fine with hot-swapping or are modified to survive it. SATA is hot-swappable (but cheaper chipsets may not have driver support), FireWire is hot-swappable, HDMI is hot swappable, DisplayPort and so on. – AndrejaKo Dec 8 '12 at 20:31
• @Vio no "2." cannot bring permanent damage, but "1." can. BTW "2." is the result of "1." – jippie Dec 8 '12 at 20:33

I can see 2 major reasons:

1. Software issues

With the CPU, RAM, PCI cards etc... they are enumerated and configured by the system upon boot-up (by the BIOS or equivalent firmware) and by the operating system (which should support hotplugging/hot-swapping).

USB, HDMI, Ethernet etc... are designed for hotplugging and thus there is appropriate support in the operating system.

2. Hardware issues

If the interface and power pins are not designed to provide hotplugging, they might see a large inrush current and some noise or distorted signals which can be harmful (e.g. cause an operating system crash) or downright destructive.

This application note explains a little more on how it's possible:

When a line card is plugged into a live backplane, the card's discharged power supply filter capacitors present a low impedance and demand a large, sudden "inrush" current. This sudden high load can cause the backplane's power supply to collapse.

If your connector only carries data (no power) then this is much less of an issue. (I thought Ethernet would be a case, but with Power over Ethernet it's still hot-swappable)

• Ethernet can carry power too and PoE is hot-swappable. – AndrejaKo Dec 8 '12 at 20:32
• Many modern OS's can cope with hot-plugging CPU's and RAM. Although it rarely happens on physical hardware, it is pretty common in Virtual Machines (or specialized server hardware). – jippie Dec 8 '12 at 20:35
• @Vio I would guess 'no' since most home PCs are not designed for hot-swapping those (it's mostly found on very high-end servers). – Renan Dec 8 '12 at 20:37
• @Vi0 yes unplugging the VGA cable. As a matter of fact (I coincidentally got to work at the board manufacturer a while later), that particular board had a redesign to prevent just that problem. But that doesn't assure other designs or other ports can't have that very problem. – jippie Dec 8 '12 at 20:48
• I would strongly advice against unplugging hardware that isn't designed for it. <SNIP> – jippie 6 mins ago – jippie Dec 8 '12 at 20:49