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A New Approach to 
Small Spacecraft On-board
Command and Telemetry
Interfacing

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Bdale Garbee, KB0G
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<bdale@gag.com>
<kb0g@amsat.org>

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AO-40 Shortly Before Launch

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Close Up of One Bay

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A Little History

Historically, all AMSAT satellites used hard wiring harnesses.

Microsat series introduced a serial multi-drop bus
	Based on Motorola AART parts
	Slow, short bus length, few channels

Recent Microsat designs used SPI for some sensors

AO-40
	Risk management led to use of a traditional harness
	CAN bus flown as an experiment	

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The Problem


Wiring Harnesses Have Issues
	Fairly static once wired
	Every module's interface is different
	Mass

Incremental Testing and Integration
	Critical to our success
	Hard to test things that require IHU involvement
	Too many tests aren't "in flight mode"

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Our Solution - Hardware


Based on Atmel CANary (8051 thingy with CAN)

	24 x 74 mm circuit board...

	12 digital outputs
	8 digital inputs
	8 analog inputs
	power switching for module
	15-pin D-shell to spacecraft, 40 pins to module
	6 bits of module address
	2 bits of mode select
	hardware watchdog

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Our Solution - Protocol


	Compatible with existing AMSAT IHU software (IPS)
	Match CPU upgrade in IHU-3 with upgraded I/O

	Very simple conceptual model
	Stateless, efficient use of CAN bus (unlike AO-40!)
	In typical use, complete state refresh every 20ms
	Allows for background use of CAN bus by experiments

	Modules support 3 modes
		normal mode
		multiplexed I/O mode using external latches
		"byte pipe" mode

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Normal Mode

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Multiplexed I/O Mode

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Byte Pipe Mode

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Can-Do!

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Can-Do!

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Can-Do!

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Status


	Designed for NA Eagle, and DL P3E and P5A 

	The hardware works

	Radiation testing gave better results than expected

	Flight firmware nearly done

	Module builder's software underway

	Two commercial small satellite companies want them!

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To Learn More

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Visit the AMSAT booth!

http://www.amsat.org/

www.gag.com/~bdale/talks