PCI Slave: The PCI Bus requires that some device on the bus generate master control signals to all the devices on the bus. Devices where high thruput is essential may desire to control the bus signals themselves. The PCI standard allows this. Devices that can generate bus controlling signals are called Bus Mastering devices. Not all devices need to master the bus, and therefore some omit the bus mastering capability. These are called PCI slave devices. Bus Mastering devices generally will have a slave mode where they do not generate bus control signals and respond to signals generated by another device.

Only one device can be mastering the bus at any time.

A PCI Slave slot or card is a slot or card that is not configured to perform bus mastering. It lacks the capability of generating data flow control signals and depends on some other device to generate them. A few motherboards have slave only slots (It's not clear why) and some warn that using a PCI bus mastering card in a slave slot may damage either the card or the motherboard, or both. There are no mechanical interlocks that prevent a bus mastering device from being plugged into a non-bus mastering slot.

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