Space Wire Physical Level Redundancy Mechanism Glenn Parker

Space. Wire Physical Level Redundancy Mechanism Glenn Parker Rakow NASA – Goddard Space Flight Center 2006 MAPLD International Conference Washington, D. C. September 25, 2006 MAPLD International Conference 1 Space. Wire 101 Seminar

Purpose • Describe method for Physical (PHY) level redundancy for Space. Wire (Sp. W) – PHY level - cable/connector – Method is backward compatible to Sp. W Standard • May be disabled – Method to be proposed to Sp. W working group for standardization • Method used on NASA and other US satellites 2006 MAPLD International Conference 2 Space. Wire 101 Seminar

Background • Redundancy may be an important part of spacecraft design 2006 MAPLD International Conference 3 Space. Wire 101 Seminar

Importance • Open systems for spacecraft should have redundancy – Requires a standardized approach for redundancy • Sp. W 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 2006 MAPLD International Conference 4 Space. Wire 101 Seminar

Link Initialization (Background) • • Sp. W initialization is important for understanding redundancy mechanism Sp. W 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 Sp. W SM has 2 end results – Connection established • – Run State Connection failed • Back to Error Reset 2006 MAPLD International Conference 5 Space. Wire 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 2006 MAPLD International Conference 6 Space. Wire 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 2006 MAPLD International Conference 7 Space. Wire 101 Seminar

Search Modes • Prim/Redun SM may be programmed to operated in 4 different modes • 2 modes compatible with Sp. W standard – “ 00” - Always Select Primary Link – “ 01” - Always Select Redundant Link • 2 search modes not compatible with Sp. W 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 2006 MAPLD International Conference 8 Space. Wire 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 • Sp. W 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 Sp. W 2006 MAPLD International Conference 9 Space. Wire 101 Seminar