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SpaceWire Physical Level Redundancy Mechanism

SpaceWire Physical Level Redundancy Mechanism. Glenn Parker Rakow NASA – Goddard Space Flight Center 2006 MAPLD International Conference Washington, D.C. September 25, 2006. Purpose. Describe method for Physical (PHY) level redundancy for SpaceWire (SpW) PHY level - cable/connector

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SpaceWire Physical Level Redundancy Mechanism

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  1. SpaceWire Physical Level Redundancy Mechanism Glenn Parker Rakow NASA – Goddard Space Flight Center 2006 MAPLD International Conference Washington, D.C. September 25, 2006

  2. Purpose • Describe method for Physical (PHY) level redundancy for SpaceWire (SpW) • PHY level - cable/connector • Method is backward compatible to SpW Standard • May be disabled • Method to be proposed to SpW working group for standardization • Method used on NASA and other US satellites

  3. Background • Redundancy may be an important part of spacecraft design

  4. Importance • Open systems for spacecraft should have redundancy • Requires a standardized approach for redundancy • SpW is only standard written for space applications • Does not currently address redundancy • PHY level redundancy should be transparent to user • Next higher layer in protocol stack should not be involved

  5. Link Initialization (Background) • SpW initialization is important for understanding redundancy mechanism • SpW initialization State Machine (SM) • Resets • Then..waits • Then..listens • If enabled - attempt to synchronize • Transmits synchronization characters • If synchronized • Exchanges flow control information • Rx buffer size • Then..exchanges Data and all other necessary characters • SpW SM has 2 end results • Connection established • Run State • Connection failed • Back to Error Reset

  6. Primary Redundant Block Diagram • Redundancy does not replicate protocol logic • Less logic • Same user (back-end) interface as if there were no redundancy • Tx FIFO & Rx FIFO • Transparent to user interface • Multiplexer (switch) to steer serial stream between cables

  7. Multiplexer Steering Mechanism • The multiplexer is controlled by a new SM • Primary/Redundant (Prim/Redun) SM • No user intervention • Prim/Redun SM keeps track of how many times “connection failed” for the Link Initialization SM • Prim/Redun SM switches multiplexer after X number of “connection failed” attempts for the same cable • Toggles cables • Primary • Redundant • X equals • 2 • 5

  8. Search Modes • Prim/Redun SM may be programmed to operated in 4 different modes • 2 modes compatible with SpW standard • “00” - Always Select Primary Link • “01” - Always Select Redundant Link • 2 search modes not compatible with SpW standard • “10” – Ping-pong every 2 attempts to connect • “11” – Ping-pong every 5 attempts to connect • Search modes are used to ping-pong between cables if connection fails to establish

  9. Summary • Redundancy is usually important for spacecraft design • Open systems need to support redundancy • Physical (PHY) level (minimum) • PHY level redundancy should be transparent to user • SpW is the only standard specifically written for spacecraft applications • JWST has implemented a PHY level redundancy mechanism • JWST PHY level redundancy mechanism will be proposed for ECSS standardization • European Cooperation for Space Standardization (ECSS) • Organization that standardized SpW

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