Assigning VME Data Bus and Interrupt PrioritiesE-2 014–001867Daisy chain A priority grant signal that originates at the start of thepriority VMEbus and propagates down the bus to the last slot. Eachboard on the bus can allow the grant to pass to the next board ortake the bus and stop the grant. The signal jumps over emptyslots.Prioritized Bus ArbitrationAs you add VME controllers to the system, you must assign a DTB priority level toeach. You set priority in two ways: either fixed or daisy chained. For each board inthe system you can assign any one of four fixed priority levels that are independentof where the board is positioned within the system. These priority levels arenumbered 0 through 3, with 3 being the highest and 0 the lowest priority. Thearbiter controls the priority hierarchy. It resides in the host and consists ofhardware and software that manages the interaction of the four priority levels.Second, each fixed level includes a daisy chained priority signal, named Bus Grant(BG). The boards gate or pass the Bus Grant signal along the bus depending on theirfixed (assigned) priority level. The fixed priority hierarchy always takes precedenceover the Bus Grant hierarchy. A description of how the Bus Grant priority chainworks is in a later section.In Data General AViiON systems, the arbiter is always associated with Slot 1 (eitherthrough physical location or logical extension via repeaters). It carries a prioritylevel of 3, the highest priority on the bus.Figure E–1 shows an example of how fixed–level priority settings and boardpositioning within the system affect overall DTB priority arbitration.BG3BG2BG1BG0Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6X XX XXXNOTE: X– Indicates that the board in this slot has been assigned this priority level.The arbiter is always 3.ArbiterFigure E–1 Example of Fixed Prioritized AssignmentsThe example in Figure E–1 shows an AViiON system with fixed priority DTBassignments. In this example, the priority arbitration sequences according to thefollowing slot order:
