12006 MAPLD International ConferenceSpaceWire 101 Seminar Glenn Parker Rakow NASA – Goddard Space Flight Center 2006 MAPLD International Conference Washington, D.C. September 25, 2006 SpaceWire Physical Level Redundancy Mechanism

22006 MAPLD International ConferenceSpaceWire 101 Seminar 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

32006 MAPLD International ConferenceSpaceWire 101 Seminar Background Redundancy may be an important part of spacecraft design

42006 MAPLD International ConferenceSpaceWire 101 Seminar 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

52006 MAPLD International ConferenceSpaceWire 101 Seminar 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

62006 MAPLD International ConferenceSpaceWire 101 Seminar 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

72006 MAPLD International ConferenceSpaceWire 101 Seminar 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

82006 MAPLD International ConferenceSpaceWire 101 Seminar 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

92006 MAPLD International ConferenceSpaceWire 101 Seminar 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