multi-master serial computer bus

http://en.wikipedia.org/wiki/Multi-master_bus

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A multi-master bus
is a computer bus
in which there are multiple bus master
nodes present on the bus. This is used when multiple nodes on the bus must/need to initiate transfer. For example, direct memory access
(DMA) is used to transfer data between peripherals and memory without the need to use the central processing unit
(CPU).

Some buses like I²C
use multi-mastering inherently to allow any node to initiate a transfer with another node.