The show makes it clear that every Stargate originally had its own DHD, located directly in front of the gate and facing it. Over time, however, some DHDs have been damaged or lost. This has been the source of plot difficulties for the protagonists on several occasions, as it is still possible to travel to a Stargate that lacks a DHD, meaning that dialing home again will be much more difficult, if not impossible. One of the primary functions of the MALP that precedes an SG team is to confirm the presence of a DHD. In the absence of a DHD, a user must select the address by manually rotating the inner ring of the Stargate, and use an external power source, as the ring will not rotate unless it is energised. Pegasus Galaxy Stargates do not have a movable ring, so manually dialing these is impossible. Travelers can also emulate a DHD through a Dialing computer as present at Stargate Command. Remote dialers have been used by several races like the Goa'uld and Asgard in various episodes. As the Stargates in Stargate Universe are a different (less advanced model) the crew of the Destiny are forced to use such a device as no planet visited so far has any variation of DHD present.