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In the prelude for SpaceX CRS-3 mission, the failure of a Multiplexer-Demultiplexer (MDM) on the International Space Station (ISS) seems to be a real concern for launching.

In the pre-launch press conference, they discussed that there are several types of MDM's that they have on station. Some seem to be internal, some external. Mike Suffredini seemed to indicate that they could use different parts of different MDM's to make up a needed type.

What are the different types of MDM's, and how compatible/pluggable are they?

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Note that this applies only to the U.S. section of the ISS.

There are 3 different types of MDMs used on the ISS.

Tier 1: Top level control. These are inside the ISS and they have 3 on board, but only one is active.

Tier 2: There are 5 different types of these, only one is external to the ISS... it is called the EXT MDM. There are 2 of every tier 2 MDM, one primary and a backup.

Tier 3: I think there are 5 of these as well, these are sensor/component level control, there are over 30 of these in the ISS if I'm not mistaken, some are inside, some outside.

From a compatibility standpoint the MDMs come in 3 different sizes, which hold 4, 10, or 16 cards. Cards and the entire assembly is easy to move around and essentially slide in and out. My understanding is that they are generally specialized cards in each MDM and cannot be swapped for other good ones, hence the backups for each system.

The one that failed recently is EXT-2 MDM (Tier 2) with a size of 16 (not all are used). A spare EXT MDM is inside the ISS, but will have to be mounted outside before it would be useful.

This article is an excellent read regarding the failure of the EXT-2 MDM and discusses the options available to the flight crew.

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At least two from the perspective of CPU. The "Enhanced MDMs" have a better CPU and better memory than do the "Standard MDMs".

There are at least four from the perspective of structure. The MDMs have cards that talk to the 1553 bus and to various "dumb" devices that don't use the 1553 bus for communication. How many cards? The standard MDMs come in three different sizes. Some have room for only four cards (an MDM-4), some for ten cards (an MDM-10), and others, sixteen cards(an MDM-16).

Another variation on the structure is the chassis, and these differ between MDMs mounted externally versus those mounted inside the ISS. The external MDMs are designed for replacement by a suited astronaut while the internal ones are designed for replacement by a shirt-sleeved astronaut. That means different attachment mechanisms.

What goes into those MDMs varies a lot. The cards on one MDM-16 will differ from those on another. (Stupid analog devices!) An MDM supposedly is an Orbital Replaceable Unit, the lowest level of object an astronaut is expected to be able to replace. That however would require keeping too many spares, one for each specific MDM. Just as hobbyists can "build" their own computers at home, astronauts can "build" an MDM in the work area. Find the right chassis, add the right cards, and presto! there's the replacement MDM.

Sources:

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