spacewire physical level redundancy mechanism

12006 MAPLD International Conference SpaceWire 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


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


Background

• Redundancy may be an important part of spacecraft design

Primary

SpWRouter

Port

Port

Port P

ort

PrimaryNode

A Por

t

PrimaryNode

B

Port

Prim

ary

Nod

eC

Port

Prim

ary

Nod

eD

Port

Primary

SpWRouter

Port

Port

Port P

ort

PrimaryNode

A Por

t

PrimaryNode

B

Port

Prim

ary

Nod

eC

Port

Prim

ary

Nod

eD

Port Prim

ary

Nod

eD

Port

Primary

InstrumentC&DHPrimary Primary

InstrumentInterfacePrimary

InstrumentElectronics

Por

t

Primary

InstrumentC&DH

Redundant Por

t

Port

Primary


Port

Primary

InstrumentC&DHPrimary Primary


InstrumentElectronics

Por

t

Primary

InstrumentC&DH

Redundant Por

t

Port

Primary


Port

Primary


Port


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


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

ErrorResetReset TxReset Rx

ErrorWaitReset Tx

Enable Rx

ReadyReset Tx

Enable Rx

StartedSend NULLs

Enable Rx

ConnectingSend FCTs./NULLs

Enable Rx

RunSend All Characters

Enable Rx

Reset

After 6.4us

After12.8us

[Link Enabled]gotNULL

After12.8us

gotFCT

[Link Disabled]

After 12.8us

ErrorResetReset TxReset Rx

ErrorWaitReset Tx

Enable Rx

ReadyReset Tx

Enable Rx

StartedSend NULLs

Enable Rx

ConnectingSend FCTs./NULLs

Enable Rx

RunSend All Characters

Enable Rx

Reset

After 6.4us

After12.8us

[Link Enabled]gotNULL

After12.8us

gotFCT

[Link Disabled]

After 12.8us


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

GENERICSPACEWIRE

LINKIP CORE

PRIMARYREDUNDANT

SWITCH

TXRX_A

TXRX_B

SWLOOPBACK

PR_MODE

TXRX

TX_DATA

RX_DATA


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


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

PRMode Description “00” Always Select Primary Link. “01” Always Select Redundant Link. “10” Ping-pong every 2 attempts to connect. “11” Ping-pong every 5 attempts to connect.


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

spacewire physical level redundancy mechanism

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