Bus grant line

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Bus grant lines are used in a number of bus architectures (for example, the UNIBUS and QBUS) for the CPU (or whatever circuitry is controlling the bus) to communicate to device controllers that their request for interrupts and DMA have been granted (hence the name).

They are often wired in series, so that one device's 'grant out' line is connected to the next device's 'grant in' line, starting the the CPU's 'grant out' line. When a device sees a 'granted' pulse on one of its grant in lines, if the device had requested a DMA/interrupt, it 'consumes' the grant pulse, and goes on to do the type of operation it requested; if it was not requesting that type of operation, it 'passes' the grant by generating a 'granted' pulse on its grant out line.

In other words, when there are conflicting requests for the bus, this kind of system prioritizes the requests based on the devices' location on the bus; the ones closest to the CPU have the highest priority.

On systems which have backplanes into which devices can be plugged, it is usually necessary to provide some means for a grant to 'jump over' an empty backplane slot; for the UNIBUS, this is done with either a G727 or a G7273 grant continuity card.

See also