Device (MSAD) strong link device used a small pellet of sensitive high explosive to trigger a larger charge of insensitive high explosive. Normally, the pellet was held away from the main charge, and was physically moved into position only when the strong link was activated by a valid input and detonated by a mechanical "slapper". The MSAD also contained a weak link: the pellet would burn or explode harmlessly in a fire when it was not in position, and the insensitive explosives could not then be detonated at all. Multiple strong links could be used in series, which, when properly designed, multiplies the safety factor.. The B61 nuclear bomb, for example, gated the trajectory strong link behind the intent strong link. Until the correct intent unique signal was sent, the trajectory unique signal would not even be presented to the trajectory strong link inputs. Unique signals Strong links implement a mechanism where only a single, unique form of energy may enter the exclusion zone. This energy is encoded as a unique signal: a sequence of "events" which must occur in a precise and preset pattern for the link to activate. This pattern is specifically designed to be extremely unlikely to occur by chance. The pattern is checked for validity by a discriminator. In some devices, known as single-try discriminators, an incorrect event pattern leads to the device becoming inoperable: the weapon cannot then be reset and fired remotely. "Multiple-try" discriminators could be reset remotely. A single-try strong link might have an event sequence of 24 events, whereas a multiple-try device would have more: the MC2969 had 47.. Unique signal patterns were always the same for a given strong link discriminator, and were not secret or classified: they were designed only for safety purposes and not security.. Each strong link had a different signal, so as to avoid the possibility of common mode failure. Unique signals were used, because it was recognised that it was impossible to fully isolate the strong link from any and all electrical sources in an "abnormal environment" (such as a disintegrating aircraft). By encoding the only valid signal as a unique pattern of information, the safety principle of "incompatibility" was introduced: the signal is "incompatible" with all other electrical energy because the information that makes up a unique signal is not present in any other components (such as signal buffers or storage). Therefore the channel over