Set priorities for daisy-chained control units

In many cases, especially when ESCON or FICON® directors are used, multiple control units are daisy-chained off the same channels either logically or physically. In this case, you do not need to prioritize all control units. (The reason is you are simply trying to map the channels for the control units. If you map the first one, the others are automatically mapped for availability. Therefore, assigning a priority for one of the control units (or groups of control units) automatically brings the others along without your assigning them priorities.)

If some daisy-chained control units have different numbers of CHPIDS going to them, separate the control units into different groups. Doing so is advantageous from an availability perspective, even though the same CHPIDs are being used. Assign the control units with fewer common channels as a higher priority (lower priority number) than control units with larger numbers of common channels.

For example, suppose control units CU1 and CU2 are daisy chained. CU1 has CHPIDs 1 and 2, while CU2 has CHPIDs 1,2,3, and 4. Assign CU1 to a priority group that is processed before the group containing CU2 (even though all CHPIDs going to CU1 also go to CU2).

Note: When you enter a priority for a control unit and other control units use the same CHPIDs, those entries are unavailable to indicate possible daisy chaining.