Synchro Chips (シンクロチップ Shinkuro Chippu) are unique Battle Chips which have the power to merge a NetNavi and a human together under certain circumstances, usually needing a Dimensional Area to do so. PET's and Dimensional Areas have a big role in the usage of the Synchro Chip.
Types of Synchro Chips
The regular Synchro Chip was invented by Yuichiro Hikari to see if NetNavis and humans can become one, which was known as Cross Fusion, but it only works inside of a Dimensional Area. Also, at first, it was noted to be dangerous, as shown by Misaki failing to fuse with PrisMan.EXE and being electrocuted. However, a Navi and a human with a high Synchronism rating can perform it and become one, and the Navi-human hybrid will gain additional powers as well. However, when the Navi and the human is not in Synchro, such as arguing with each other, the Synchro will be broken. Also, people with the Crest of Duo can Cross Fuse thanks to the fact that it naturally increases the base Synchronism rate.
Dark Synchro Chip
Dark Synchro Chips where invented by Dr. Regal by taking the Synchro Chips that ShadeMan.EXE stole from SciLab, and combined it with Dark Chips. The Dark Synchro Chip forcibly increases the Synchro rating. However, because it was crafted from a Dark Chip, the same negative effect of a Dark Chip (becoming evil and more powerful) will corrupt the human and the NetNavi, as shown by Misaki with PrisMan.EXE in Episode#34 (Japanese numbering) of Axess. However, Ms. Yuri and Regal where unaffected. It most likely is because they were a probe of Duo, since Dark ProtoMan stated on Episode#48 (Japanese numbering) that one of the advantages of being a Probe of Duo was that the Dark Synchro Chip despite forcibly increasing the Synchronism rating, did not affect the consciousness of the users. In Dr. Regal's case, though, it's intriguing to note that once he's performed Cross Fusion, his personality becomes more violent and arrogant than it was before. It might only be a feeling of utter overconfidence or of thinking that him being a Probe of Duo was fitted to rule the Earth, as he declares in Episode#51 (Japanese numbering).
Rush Synchro Chip
The Rush Synchro Chip was invented by ShadeMan.EXE, and later replicated by Mr. Famous. ShadeMan created this by forcibly combining Rush with a Synchro Chip, which caused Rush to be traumatized about the event. Mr. Famous managed to recreate the Rush Synchro Chip, and is now used by Mayl Sakurai, although Rush dislikes to be used (but still allows itself to become the Battle Chip, although reluctantly). The Rush Synchro Chip, when used solely by a NetNavi, is shown to be able to materialize into the real world without the need of a Dimensional Area. When used by a human and a NetNavi to CrossFuse, however, it instead increases the Synchro rating. Also, the user gains the Rush Hole ability, being able to send enemies and viruses back to the CyberWorld.
Reverse Synchro Chip
The Reverse Synchro Chip was invented by Lord Wily. Instead of Cross Fusing with NetNavis in the human world, it instead sends the CrossFused hybrid to CyberSpace instead. Otherwise, it's no different than a regular Synchro Chips, although it was incomplete. Since the chips were incomplete, it could actually delete/erase the NetNavi/human.
All Synchro Chips share a common weakness: if the crest (the insignia of the Navi) is damaged in any way, Cross Fusion is instantly disabled. This is shown in the simultaneous damage of both Crossfused MegaMan's and Crossfused PrisMan's crests, the destruction of Ms. Yuri's crest by Dark ProtoMan, the damage inflicted by an Netopia mecha on Crossfused MegaMan during the events of The Program of Light and Dark, and a fight with CosmoMan during the events of Stream. Lan was using a regular Synchro Chip while both Misaki and Ms. Yuri were using Dark Synchro Chips.