Dial Home Device (DHD)
The Tau'ri-named Dial Home Device, or DHD, was built by the Ancients as a companion component to the Stargate. The DHD is the control mechanism for the Stargate; it enables a traveler to dial a destination Stargate address and request activation of the wormhole. The DHD directs the Stargate's inner ring to spin to the desired coordinates and assures that the address is valid and that all conditions are acceptable before allowing the wormhole to engage. If those conditions are satisfied, the DHD sends a power signal to the Stargate, causing the Stargate to use its stored energy to open the wormhole.
Having a functioning DHD is not an absolute requirement to operate a Stargate. The SGC operates its Stargate with a dialing computer that performs many of the DHD's functions. Also, in extreme circumstances, a traveler may dial an address manually by spinning the inner ring of a gate. However in this case, the safety features provided by the DHD are bypassed.
Using a DHD
A DHD has 38 unique glyphs arranged in two concentric circles around a central activation dome which covers the master control crystal. A traveler presses a series of glyphs, in order, until the entire address of the destination has been selected. An address usually consists of six glyphs that represent a point in three-dimensional space. Some special addresses outside of our galaxy have a sort of 'area code' as an additional glyph after those of the address; it represents a distance calculation. After entering the address, the traveler presses the glyph for the point of origin of his or her current location. As the traveler is dialing, the DHD sends signals to the Stargate, and its inner ring spins so that each pressed glyph aligns at the next available chevron on the outer ring. Progress is indicated by the chevrons lighting as each one locks. Once the traveler is ready to go, he or she pushes the activation dome in the center of the DHD. If all conditions are good, the Stargate's wormhole bursts forth then settles into a stable event horizon which the traveler may enter.
If the DHD detects a problem, the wormhole will not form and the lighted chevrons will de-energize. The following detected problems will cause the wormhole to fail to appear:
- The destination Stargate is already active with an incoming or outgoing wormhole
- The destination Stargate is buried or otherwise blocked so completely that there is no space for the wormhole to form
- Other conditions affecting safety are detected, such as the wormhole being predicted to pass through a sun
The DHD works within the Stargate network to correct the glyphs' coordinates for stellar and planetary shifts that over time would make the original constellations incorrect (1.01 "Children of the Gods Part 1").
The Stargate network is just that: a network. DHDs may communicate with each other over this network, for example, to propagate changes to their Stargate's coordinates that other DHDs need to know about. Such an event is called an automatic correlative update and occurs approximately every 200 years. This communication capability had disastrous consequences when the SGC's Dr. Felger used the dialing computer to scramble the coordinates of the DHD on one of Ba'al's principal mining planets. It worked, but Ba'al used the Stargate network to propagate the virus to all DHDs, using the correlative update logic. Carter and Felger had to reset the mining planet's DHD and propagate the virus-free logic throughout the system in order to fix the problem (7.09 "Avenger 2.0").
The inter-DHD communication and Ba'al's knowledge of it were critical to the final destruction of all Replicators in our galaxy. Once Carter and Jacob configured the Ancient's weapon with an energy beam that would only kill Replicators, Ba'al showed them how to force the Dakara DHD to dial all other DHDs in the galaxy simultaneously. They then sent the energy beam through the wormholes and destroyed the Replicators (8.17 "Reckoning Part 2").
If a DHD is damaged or destroyed while its Stargate's outgoing wormhole is engaged, the Stargate may lose power and sever the wormhole prematurely. Depending on the timing, the receiving Stargate will store the energy signatures of any matter that was en route inside the wormhole. A DHD on the receiving Stargate can compensate for the loss of power and allow the gate to finish the matter reintegration process (5.14 "48 Hours").
During the process of dialing an address, even after the Stargate's inner ring has begun spinning, any incoming wormhole will override the dialing sequence and form an event horizon. This will reset the outgoing dialing sequence and prevent any further dialing. This feature was often used by the Goa'uld to prevent their enemies from dialing out (2.17 "Serpent's Song" and others).
If there are two Stargates on a planet, only one may be active at a time. If only one has a true DHD, that one will become the primary Stargate and prevent the other from operating if both are dialed at precisely the same time (4.07 "Watergate"). Also, in the event of a malfunction, travelers might be sent to the second Stargate, since it has the same address (1.18 "Solitudes").
Like the Stargate, the DHD is made of naquadah. It has control crystals inside and is powered by an internal naquadah power source (1.11 "The Torment Of Tantalus" and others). A DHD has a master reset switch to interrupt the power and 're-boot' the device (1.18 "Solitudes").
The SGC Dialing Computer
Before they knew of the existence of DHDs, US military personnel reverse engineered the interface to the Stargate and created a computer program to read or emulate some of the signals that a DHD exchanges with a Stargate. Instead of lighted glyphs, the dialing computer uses a front-end keyboard and monitor to enable input of the Stargate address and monitor the progress of the dialing sequence. It took fifteen years and three supercomputers to 'MacGyver' the system to the point where it was minimally functional (1.01 "Children of the Gods Part 1"). During development, the scientists generated a series of instructions based on electrical impulses to which the gate crystals would respond. These signals and their purpose were found mostly by trial and error over a great deal of time (5.14 "48 Hours").
At first the ride through the SGC's gate was rough and the traveler arrived covered with frost (1.01 "Children Of The Gods Part 1"). However that was corrected as the planetary drift calculations of the computer were made more accurate (5.05 "Red Sky"). The computer was also tuned to dial the gate faster than it did at first; now the dialing speed is nearly as fast as a DHD (2.17 "Serpent's Song"). The SGC presumably learned more about the internal workings of a DHD when O'Neill used his Ancients knowledge to draw detailed plans of one in order to suggest a repair; this might have contributed to improvements to the dialing computer (2.16 "The Fifth Race").
Still, five years into the Stargate Program, McKay noted that the dialing computer ignored 220 of the 400 signals that the gate could emit during any given dialing sequence (5.14 "48 Hours"). That was why Carter and her team had to override some of the dialing protocols of the gate to get a lock on K'Tau. The Stargate system sensed a problem, but the dialing computer did not have a complete interface, so it did not report what the problem was: that the current alignment of the two planets would cause the wormhole to travel through K'Tau's sun, causing catastrophic changes (5.05 "Red Sky").
Also, the Earth Stargate draws 10% more wattage than off-world gates, and Carter speculated that this is because the DHDs are somehow producing a purer, more efficient form of power than the SGC system is able to deliver (7.17 "Heroes Part 1").
The interface includes connections to Stargate internal sensors, for example to measure energy signatures within the wormhole; the computer can store them for later study (9.09 "Prototype" and many others). This sort of graphical analysis of what is transpiring within the wormhole shows one advantage the dialing computer has over the 'black box' nature of the DHD. In fact the positive attributes of the computer are significant enough that, while it has a functioning DHD, the Alpha Site uses a dialing computer to operate its Stargate (8.11 "Gemini").
The SGC dialing computer has been used to dial locations outside of our own galaxy. In both cases, eight glyphs were used to define the address, and an exceptional level of power was required to form a stable wormhole to the unimaginably distant destination (2.16 "The Fifth Race", Atlantis 1.01 "Rising Part 1"). Presumably a DHD could do the same thing, but that capability has never been shown.
Earth's DHDs and Stargates
The history of the location and usage of the Earth's DHDs and Stargates is elaborate. Here is a timeline:
Some millions of years ago, perhaps as much as 50 million years, the Ancients placed a Stargate and DHD in Antarctica (6.04 "Frozen").
Thousand of years ago, the Goa'uld Ra left a Stargate and DHD in Giza, Egypt (Stargate: The Movie).
The Giza Stargate was unburied in 1928 and taken for study by the Americans (Stargate: The Movie).
The companion DHD for the Giza Stargate was unburied by the Germans earlier, in 1906. It was taken from them by the Russian Red Army at the end of World War II (5.08 "The Tomb").
O'Neill and Carter accidentally found the Antarctic Stargate and DHD in 1997 (1.18 "Solitudes"). The SGC put both devices into storage at Area 51, but the NID secretly began using both, until they were discovered and stopped the first time about a year later (2.14 "Touchstone").
The Antarctic DHD died shortly after it was brought back from Antarctica to Area 51, presumably after the NID had stopped using it. It was used a few more times and then ran out of energy, which was an indication of how old it was (6.04 "Frozen").
Replicators were overrunning Thor's ship Beliskner, in orbit around Earth with O'Neill, Carter, and Teal'c aboard. O'Neill beamed the Giza Stargate from the SGC onto the ship and used it to dial out and escape. Though there was no DHD aboard, there was power to dial the gate manually and since the ship was near Earth, they could use Earth as the point of origin. The Giza Stargate then crashed into the Pacific Ocean along with the destroyed Beliskner and was assumed lost by the US Government (3.22 "Nemesis Part 1").
In fact, the Russians saw the Beliskner entering the atmosphere on their radar systems. They went to investigate and recovered the Giza Stargate after an extensive sweep of the ocean floor. By combining this Stargate with the Giza DHD they already had, they were able to start their own gate travel less than one month later. The SGC discovered this a few weeks later, when their own gate failed to lock because it was in use by the Russians (4.07 "Watergate").
When Teal'c's energy signature became trapped in the SGC gate with no way to reintegrate, the Tok'ra advised Earth that the only way to save him would be to attach a DHD to the gate. Its internal systems would compensate for the loss of power and allow the gate to finish the reintegration process. Daniel went to Russia and in the end they agreed to loan the DHD to the U.S. The Goa'uld possessing Adrian Conrad further advised the SGC on what to do to save Teal'c — remove the master control crystal. Unfortunately the actions, while successful, resulted in the DHD exploding in the process (5.14 "48 Hours").
Under attack by Anubis, the Antarctic Stargate was removed from the SGC and sent into space, where it was destroyed. The SGC then leased the Giza Stargate from the Russians in exchange for money and blueprints for the X-302 and 303 (6.02 "Redemption Part 2").
At this time, the Giza Stargate is the one in use by the SGC, and Earth has no functioning DHDs.
It stands to reason that not all Stargates and DHDs have exactly the same glyphs. First, since each point of origin glyph is unique, if there were only one 'model' of gate, then there would be only 39 unique Stargates in the network. We know there are thousands, perhaps millions of Stargates in our galaxy, so the glyphs must not be the same on every Stargate. Second, of course, the idea that glyphs represent constellations visible on the planet the gate is on means that every gate would have completely different glyphs. After all, the star constellations are completely dependent on one's point of view.
This variation implies that a DHD is built to match a Stargate and cannot be interchanged at will. Perhaps there are procedures to modify a DHD's internal configuration and glyph panels so that an interchange is possible after some effort.
The variation in gate and DHD symbols also implies that not every gate/DHD can dial every other gate. That is, if the originating gate does not have all of the symbols required for the destination address, then a direct connection cannot be made. An intermediate stop would be necessary in that case, to find a Stargate and DHD with all necessary glyphs present.
A Stargate has 39 glyphs including a point of origin, while a DHD has only 38, also including a point of origin. This implies there is one symbol on the Stargate that can never be dialed by its DHD. Perhaps it serves a different function yet to be revealed.
Each DHD glyph lights up as it is pressed. The glyphs stay lighted after the wormhole engages, giving some indication of what address was just dialed. However the order of the glyphs is not indicated by the lighted panels. So, it is not possible for someone coming upon a just-dialed gate to know precisely what address was dialed, but only which glyphs were involved. As McKay noted, the number of possible addresses represented by a single set of six glyphs is 6!, or 720 (Atlantis 1.02 "Rising Part 2").
The number of possible six-glyph addresses that can be dialed by a DHD, using each glyph at most one time, is on the order of 1.99 billion, or 38!/(38-6)!.
- Stargate: The Movie
- 1.01 "Children Of The Gods Part 1"
- 1.11 "The Torment Of Tantalus"
- 1.18 "Solitudes"
- 2.14 "Touchstone"
- 2.16 "The Fifth Race"
- 2.17 "Serpent's Song"
- 3.22 "Nemesis Part 1"
- 4.07 "Watergate"
- 5.05 "Red Sky"
- 5.08 "The Tomb"
- 5.14 "48 Hours"
- 6.02 "Redemption Part 2"
- 6.04 "Frozen"
- 7.09 "Avenger 2.0" Episode Guide
- 7.17 "Heroes Part 1"
- 8.11 "Gemini"
- 8.17 "Reckoning Part 2"
- 9.09 "Prototype"
- Stargate Atlantis
- Gen. Jack O'Neill
- Dr. Daniel Jackson
- Lt. Col. Samantha Carter
- Jacob Carter
- Jay Felger
--Michelle 12:48, 6 Nov 2005 (PST)