Modularity is undoubtedly a good thing, especially when it allows you to upgrade equipment you bought some time ago thinking it might never support the very feature you're now able to upgrade to. So it is with the Ninja V and the just-announced SDI module (the AtomX).
The Ninja V is the first product with a new architecture that includes a proprietary video processing chip (AtomIC) and a fast expansion port. Based on the industry standard of PCIe, this is able to happly support 12G SDI and - of course - everything slower.
The module is capable of receiving and outputting 12G which allows 4K at up to 60fps. The modules are bi-directional, selectable in software.
For almost its entire history, Atomos has been planning a modular system, using stackable units, held in place by a battery slot. You'll be able to stack multiple modules, limited only by the ultimate bandwith of the expansion port and the internal processing avaialble to it inside the Ninja V.
The module is the size of an NP type battery and takes standard, full-sized connectors. Either port can be configured to transmit and receive signals at 4Kp60 or 2Kp240. Playout can be over both HDMI and SDI outputs. It can also output from both SDI ports at the same time.