You can enable VTAM® to define dependent LUs dynamically. One way to do this is to use the IBM-supplied selection of definitions for dependent LUs (SDDLU) exit routine. With this exit, VTAM can define LUs dynamically when it receives information from the PU that the device is powered on, rather than when the major node is activated. The dependent LUs must be attached to the host through PUs that support Reply/PSID NMVTs, request units that are used for dynamic definition of dependent logical units. For example, you can use a 3174 cluster controller because it supports these request units.

VTAM uses model LU definition statements to build LU definitions. Define model LU definition statements based on information the PU sends to VTAM about the logical unit device. The default information that the PU sends is the seven-character machine type and model number. When the PU is activated or when a device powers on, the PU sends a Reply/PSID NMVT request unit to VTAM, which can trigger VTAM to build or change LU definitions. This NMVT contains the local address of each LU, a power indicator (on or off), the machine type and model number of the device, and optionally other device-dependent information needed to define the LUs. VTAM uses this information to choose an appropriate model LU definition statement to build an LU definition.