MDMs are installed in a relatively simple process, via two captive fasteners on their left and right sides, and a center jackscrew to hold the MDM enclosure in its location. There are three different sizes of MDMs – MDM-4, MDM-10, and MDM-16 – which simply means that some MDMs are sized for 4 circuit cards, some for 10 circuit cards, and some for 16 circuit cards, with each size having a wider enclosure to house their specific number of cards. Different types of cards inside the MDMs perform different functions. In terms of physical hardware, while all MDMs have common features, their construct varies depending on their type, which is governed by the fact that some MDMs need more circuit cards than others in order to operate, since some control more complex systems than others. The N1, PVCU, and S0 MDMs are located outside the ISS, while the AL and LA MDMs are located internally. The different types of Tier 3 MDMs are Node 1 (N1), Airlock (AL), Lab (LA), Photo Voltaic Control Unit (PVCU), and Starboard Zero (S0), with each providing sensor processing to their specific modules. Tier 3 MDMs, also known as User MDMs, are responsible for data processing for the thousands of sensors aboard the ISS. With the exception of the EXT MDMs, all are located inside the ISS, and all are single-fault tolerant, meaning two are available, with one as primary, and the other as back-up. The different types of Tier 2 MDMs, known as External (EXT), Internal (INT), Payload (PL), Guidance, Navigation & Control (GNC), and Power Management Control Unit (PMCU) MDMs, all provide control for the individual systems to which they are assigned. Tier 2 MDMs, also known as Local MDMs, are responsible for system-specific control and data processing. They are two-fault tolerant, meaning three are available, with one as primary, and two as back-ups. They consist of only one type of MDM, called Command and Control (C&C) MDMs, which are located inside the ISS. Tier 1 MDMs, also known as Control Tier MDMs, are responsible for overall control of the entire US segment of the ISS. Within these tiers, different types of MDMs exist, each designed to control a unique system. ISS MDMs are organised in a hierarchical command structure known as Tier 1, Tier 2 and Tier 3, with each tier having its own specific purpose. The MDMs contain all the software needed to control these systems, hence why they can essentially be thought of as computers. The ISS MDMs are part of the Command and Data Handling (CDH) system, which controls all major functions of the US segment of the ISS, including power generation and distribution, attitude control, environmental control, communications systems, and monitoring of scientific payloads. Despite the challenges, SpaceX’s Dragon CRS-3 mission – which could have been impacted by the MDM failure – was still on for Monday, prior to its scrubĪ Multiplexer/Demultiplexer (MDM) is essentially an electronics unit that sends and receives multiple streams of data, and thus all incoming and outgoing data to and from the ISS is routed to its correct destination via an MDM. The International Space Station (ISS) suffered a failure of an external computer known as a Multiplexer/Demultiplexer (MDM) on Friday night (11 April), which will require a spacewalk on April 22 to replace in order to restore vital redundancy to the station.
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