Space Wire Physical Layer Issues Shaune Allen NASA

Space. Wire Physical Layer Issues Shaune Allen NASA Goddard Space Flight Center 2006 MAPLD International Conference Washington, D. C. September 25, 2006 MAPLD International Conference 1 Space. Wire 101 Seminar

Problem Statement § Chapter (Clause) 5 Of the Space. Wire Standard (ECSS-50 -E-12 A) provides a good basis for the Space. Wire Physical Layer § Sp. W issues should be carefully considered by the designer to insure requirements are understood and met 2006 MAPLD International Conference 2 Space. Wire 101 Seminar

What are the Requirements? § Jitter - not specified in the Sp. W Standard - Many use the recommended maximum 20% tui • National Semiconductor AN-1088 • Actual application may require a different requirement - Eye Pattern measurement § Skew - Pair to Pair: <0. 15 ns/m - Within Pair: <0. 1 ns/m § Bit Error Rate - not specified in the Sp. W Standard - often <1. 0 E-12 using worst case pseudo-random sequence - Actual application may require a different requirement 2006 MAPLD International Conference 3 Space. Wire 101 Seminar

Skew and Jitter 2006 MAPLD International Conference 4 Space. Wire 101 Seminar

Contributors to Skew Connector Skew Encoder Driver PCB Skew Cable Skew Connector Skew Driver Decoder PCB Skew 2006 MAPLD International Conference 5 Space. Wire 101 Seminar

What are the Requirements? § LVDS Threshold Levels - ± 100 m. V or minimum 200 m. V eye height § LVDS Common Mode Range - not specified in the Sp. W Standard - Often ± 1 V - Actual application may require a different requirement 2006 MAPLD International Conference 6 Space. Wire 101 Seminar

LVDS Threshold Levels 2006 MAPLD International Conference 7 Space. Wire 101 Seminar

Common-Mode Range 200 m. V 2. 4 V VOS = 1. 2 V 200 m. V 0 V § LVDS signal swing should remain within the 0 -2. 4 V range § Check your with LVDS manufacturer for specific range for your application § Common-mode range dependent upon length of cable assembly and data rate 2006 MAPLD International Conference 8 Space. Wire 101 Seminar

What are the Requirements? § Cable Length - 10 m suggested (should vs. shall) - Up to 20 m shown to operate at applicable rates - Actual application may require a different requirement § Wire Gauge of signal wires - AWG 28 consisting of seven strands of 36 AWG wire - Current petition to add AWG 26 version to the Standard § Differential Impedance - 100 ± 6 - Standard requirement • Make sure design, analysis, and test supports this requirement 2006 MAPLD International Conference 9 Space. Wire 101 Seminar

Cable length vs. data signaling rate § ANSI/TIA/EIA-644 -A offers the following guidelines - The maximum permissible length of cable separating the generator and the load is a function of • • data signaling rate influenced by the tolerable signal distortion the amount of longitudinally coupled noise common potential differences introduced between the generator and the load circuit commons • cable balance 2006 MAPLD International Conference 10 Space. Wire 101 Seminar

Cable length vs. data signaling rate § ANSI/TIA/EIA-644 -A offers the following guidelines - Eye patterns are recommended to determine the amount of jitter at the load at the application data signaling and comparing that to system requirements • Typically, maximum allowable jitters tolerances range from 5% to 20% depending upon actual system requirements • Testing should be done in the actual application if possible, or in a test system that models the actual application as close possible • Parameters that should be taken in account include - balanced interconnect media characteristics Termination protocol and coding scheme worst case data patterns (I. e. pseudo random). The generator / receiver manufacturers and also the media manufacturers should provide additional guidance in predicting data signaling rate versus cable length curves for a particular generator / receiver and a particular media as this relationship is very dependent upon the actual characteristics of the selected devices and media. 2006 MAPLD International Conference 11 Space. Wire 101 Seminar

Eye Patterns 2006 MAPLD International Conference 12 Space. Wire 101 Seminar

Eye Patterns 2006 MAPLD International Conference 13 Space. Wire 101 Seminar

Eye Patterns 2006 MAPLD International Conference 14 Space. Wire 101 Seminar

What are the Requirements? § Insertion Loss (or Attenuation) - not specified in the Sp. W Standard - Consider the requirements of the application § Return Loss - not specified in the Sp. W Standard - Consider the requirements of the application § Shielding Effectiveness - not specified in the Sp. W Standard - Consider the requirements of the application § Near End Crosstalk (NEXT) - not specified in the Sp. W Standard - Consider the requirements of the application 2006 MAPLD International Conference 15 Space. Wire 101 Seminar

What are the Requirements? § Space. Wire Standard specifies MDM-9 connector - Mismatched to 100 differential impedance of Space. Wire cable - Mechanical issues (mate/demate) - Inner shields (signal ground) pass through a single pin - Unable to pass inner shields through bulkheads - Near-end Crosstalk (NEXT) 2006 MAPLD International Conference 16 Space. Wire 101 Seminar

Suggested Changes to Standard § Add 26 AWG version of Sp. W cable to detail spec § Allow use of twinaxial connectors 2006 MAPLD International Conference 17 Space. Wire 101 Seminar

Space. Wire Testing § Bit Error Rate Measurement § Shielding Effectiveness § Near End Crosstalk (NEXT) § Ground Bounce Susceptibility § LVDS Common Mode Range § Common Mode Rejection 2006 MAPLD International Conference 18 Space. Wire 101 Seminar