Dial-Home Device
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In the science fiction television Stargate universe, a DHD (for Dial-Home Device) is an Ancient machine used to control a stargate. DHDs are pedestal-shaped with a round inclined control panel on top consisting of two concentric circles of "keys" and a translucent red hemisphere in the center; the keys represent the symbols on the rim of the Stargate and the central hemisphere serves as an "enter" key to activate the Stargate once a destination has been dialled. The DHD also provides power for the Stargate and appears to have a complex programming interface with it that is not normally needed by the operator.
It appears that originally every Stargate had its own DHD, located directly in front of the Stargate and facing it. However, over time some DHDs have been damaged or lost. This presents a difficulty for Stargate travellers, as it is still possible to dial in to a Stargate that lacks a DHD but dialling out again is much more difficult. Several episodes have hinged upon SG teams becoming stranded on worlds without functioning DHDs, having to improvise lightning rods or other such power sources and manually dialling the Stargate's symbol ring. One of the primary functions of the MALP that is sent to new Stargates before any SG team is to confirm the presence of a functioning DHD.
According to Dr. Zelenka, dialing an address leaves a small imprint on the control crystals of the DHD, so about fifty addresses can be recovered from a DHD from someone with the proper equipment; however, this gives no indication of the order in which the addresses were dialed and no guarantee can be made as to the accuracy of the recovered addresses. ("The Lost Boys")
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Earth's DHDs
Earth's Stargate was missing its DHD when first discovered, requiring Stargate Command to develop its own native dialling system (or 'MacGyver our own' as Sam puts it in "Children of the Gods"). This resulted in Stargate misbehavior from time to time, since most of its detailed interfaces and safety features could not be reverse-engineered.
Earth's DHD had not been destroyed, however, merely lost; the Nazis removed it from Egypt during World War II and the Soviet Union subsequently captured it at the end of the war. It passed into the possession of the Russians after the Soviet Union's fall. After the Russians recovered the American primary Stargate from the Pacific Ocean when it was lost in the crash of Thor's starship Beliskner, they set up their own Stargate exploration program using the DHD to secretly "override" the backup Stargate the SGC was using whenever Russian SG teams were scheduled to dial back to Earth. The backup Stargate's DHD was still with it when it was found in Antarctica, and was used briefly by NID operatives to conduct covert technology raids through the backup Stargate, but it has since run out of power and ceased to function. ("Watergate") The Antarctic gate and DHD are thought to have been the oldest known, possibly among the earliest that were constructed by the Ancients.
The American Stargate program eventually suffered a mishap that their makeshift dialling interface was unable to correct, with Teal'c becoming "trapped" in their Stargate's data buffer after the wormhole connection was severed when a ship destroyed the stargate that the Earth stargate was connected to ("48 Hours"). The Americans eventually made a bargain to exchange some of the information their more-successful Stargate program had gained for the Russian DHD, which was able to retrieve Teal'c alive. The DHD was (spectacularly) burned out in the process, however, leaving Earth with no remaining known DHDs.
Pegasus galaxy DHDs
In the series Stargate Atlantis, an expedition to the Pegasus galaxy discovers another Stargate network along with extensive remnants of Ancient civilization. Conventional DHD pedestals are found in most worlds of the Pegasus Galaxy, with mainly cosmetic differences from DHDs in the Milky Way. The primary differences are that both the keys and the central hemisphere in the Pegasus Galaxy are blue, the sound produced by dialing a Pegasus DHD is more of a buzzing than a swishing, and that gate-dialing is much faster, likely due to the fact that, unlike the Stargate in America and its galaxy, these Stargates do not dial manually. In addition, the DHD keys are made of a reflective, glass-like material. The Wraith also travel through Stargates in small spacecraft called Wraith Darts and have some means of remote-dialling them in a manner similar to Ancient ships.
Atlantis DHD
The Atlantis DHD is more like the Earths dialing computer than an actual DHD, it looks more like a set of crystal panels. Consequently, the Atlantis DHD also has an extra control-crystal allowing the dialing of an eighth chevron during the dialing sequence and is the only DHD in the Pegasus galaxy capable of dialing Earth. This was probably a security measure set up by the Ancients so that their evacuation site (Earth) was not taken control of by the Wraith.
Puddle Jumper DHD
The Puddle Jumper DHDs are very similar to the Atlantis DHD with the exception of the extra control-crystal. The ships DHD is used for travel through an orbital stargate possibly in conjunction with a Stargate Power Node since orbital Stargates have no constant DHD. These DHDs seem to be tailored to their counterpart galaxy since the Puddle Jumpers from Atlantis have the Pegasus set of glyphs where as the Gate Ship in Stargate SG-1 had the Milky Way set.
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