Avenger 2.0 - S7-E9
Corrected entry: The whole premise for this episode is that we can transmit or implant a virus into a DHD which will scramble the coordinates, so that the symbol on the DHD will not correspond to the same symbol on the gate. Because of this, SG-1 accidentally manage to disable every gate in the gate system. Has everyone forgotten how to manually dial a gate? As the virus only infects the DHD, you can still dial manually, as the DHD is not required when dialing manually.
Correction: The gate and DHD are connected, the DHD is mostly a controller and both together operate as a computer, in this case, it is not the DHD that is scrambled, but it is the DHD that has scrambled the gate. This is why Earth's gate was not scrambled, it has no DHD.
Sol Parker