The channel-enable switches on a communication controller panel (or the MOSS console for the 3720 and 3745) determine whether VTAM® can activate a channel link and link station during an NCP activation. For a communication controller that is both channel-attached and SDLC-link-attached, these switches also determine whether an NCP can be loaded over a channel or an SDLC link. If any channel adapters are enabled on the communication controller, the remote program load (RPL) feature of the communication controller is disabled, and the communication controller can be loaded only over one of these channels. If none of the channel adapters are enabled, the communication controller can be loaded only over SDLC links defined as being capable of IPL (previously described under Selecting a load station).

All channel adapters should be kept enabled during normal operation with channel-attached NCPs. This is because a channel-link station activation fails (with the channel appearing to be "not operational") if the channel adapter for the channel VTAM is attempting to use is disabled. VTAM does not recover the use of the channel (even if the channel adapter is later enabled) unless the NCP is deactivated and reactivated. If all channel adapters are enabled, the SDLC links are still available for normal use after the NCP is loaded or if the NCP has been previously loaded. The channel-enable switches should be turned off only when a communication controller must be loaded over an SDLC link (such as when the channel-attached host has failed along with the NCP).

Figure 1 shows an example of an NCP that is both channel-attached and SDLC-link-attached to a VTAM host. For this example, assume that NCP1 is active and that NCP2 is inactive and requires loading of its NCP load module.

Figure 1. Loading over a channel

VARY NET,ACT,ID=NCP2,U=0C2,RNAME=LINKSTA1

VTAM then attempts to activate the adjacent link stations 0C2-S and LINKSTA1. Activation of LINKSTA1 in NCP1 is delayed because NCP1 receives no response to link-level commands sent on the SDLC link to NCP2 communication controller before and during the loading of NCP2 over the channel. Because the channel adapter is enabled, activation of 0C2-S proceeds, and VTAM begins the loading of NCP2 over the channel. After the load completes and LINKSTA2 is activated, contact is established with NCP1, and the activation of LINKSTA1 completes.