DIAL-HOME DEVICE

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In the science fiction 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 (Milky Way) or blue (Pegasus) hemisphere in the center; the 38 or 36 keys represent the corresponding symbols (or glyphs) on the rim of the Stargate with the central hemisphere serving to engage the Stargate. When a DHD is present the gate will not simply activate even if the inner ring is spun in the correct combination as shown in the Stargate film. This likely serves as a fail safe to ensure that the correct address was entered. It also serves to allow for "quick dialing" which allows the gate to activate without waiting for the inner ring to spin. 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.

DHDs are not the only methods of dialing Stargates. There are 2 other ways that a Stargate has been seen to be dialed:

  • By Dialing Manually
     If no dialling system is present, a user must manually select the address. On a Stargate of Milky Way design, this involves using sheer force to rotate the inner ring to select each of the seven symbols located on it (like rotating a colossal combination lock). A manual power-source is also required; the element the gate is composed of is described as a superconductor to which electricity can be fed directly, and so lightning strikes have been shown to be minimally sufficient. Pegasus Galaxy Stargates do not have a movable ring and so manually dialing these may be impossible.
  • With an Alternate Dialer
     If no DHD is present, travellers will be forced to emulate one. The technology within a DHD is described as far superceding Earth technology, making DHD emulation particularly difficult. Examples of alternate dialers include:
    • The Stargate Command Dialing Computer: this is the technology built in the film by humans to make use of the gate. Through trial and error, the SGC managed to discover that a Stargate can be communicated with by means of electrical impulses. The SGC dialing computer uses these impulses to communicate with the gate, allowing for a series of gate diagnostics, as well as the means for dialing the gate.
    • The Atlantis Gate dialer: essentially a Puddle Jumper DHD; however, it appears to have extra features, like blocking out certain gate addresses.
    • Remote dialers: these can be held in the hand or worn on the wrist, and have been used by the Goa'uld and Asgard in various episodes, as well as by the character Cassandra in the episode "1969". The Nox character Lya was seemingly able to engage the Stargate through her own powers, but perhaps had an unseen remote dialer.
EARTH & THE MILKY WAY 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. 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 dialing 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. 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.

The Milky Way DHDs have sparked numerous discussions as the design and operation of the device has resulted in contradiction throughout the SG-1 series. In the Milky Way, the Dial-Home-Device contains 38 of the 39 symbols on the Stargate, meaning there is always a missing glyph on each DHD. This missing glyph however is not the point of origin for the planet. It has been confirmed that the missing glyph on numerous DHDs differs based on where each Stargate is positioned in the galaxy. The glyph that is hidden under the pedestal of the Stargate, unseen along with the two chevrons, cannot be dialed by that DHD. This means that only certain addresses can be reached from certain positions in the galaxy. The only way to intervene and reach all destinations in the Milky Way is to manually dial the gate and use an external power source, for example: the SGC.
 

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 due to the fact that the Pegasus DHDs are newer, and obviously feature a different coordinate system specific to that galaxy. 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 on Earth 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 darts and have some means of remote-dialling them in a manner similar to Ancient ships.
 
ATLANTIS DHDS
The Atlantis DHD is more similar to the Earth's dialing computer than an actual DHD, and looks more like a set of crystal panels. A similar DHD is also used on Puddle Jumpers.

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 not everyone could dial into another galaxy, such as the Wraith from dialing into Earth.
 

PUDDLE JUMPER DHDS
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. However, it may be that the Jumpers possess a sort of 'universal' DHD that tailors itself to the gate system in question, as a Jumper from Atlantis has been seen activating a gate from the Milky Way.