[aprssig] APRS on ECHO

Jason Winningham jdw at eng.uah.edu
Wed Jun 29 08:49:32 CDT 2005

I just noticed this from "AMSAT NEWS SERVICE BULLETIN 177.S1 FROM AMSAT 
SILVER SPRING, MD, JUNE 26, 2005" posted to rec.radio.amateur.space.  I 
thought I'd post this to the list since APRS via ECHO has been 
discussed recently.


> From the Satellite Software Development Team:
> Due to restrictions in the Satellite, there can be no digi 
> functionality
> on Echo at this time.
> Explanation:
> Random noise from a receiver that is fed through the demodulator and
> into the SCC (a sort of smart UART) will cause interrupts to be sent to
> the CPU. With Echo having 4 or 5 open receivers the number of 
> interrupts
> was enough to cause the CPU difficulties in getting normal processing
> done in a timely fashion.  Basically, the CPU was locked up trying to
> handle the interrupts. Echo is operating its SCC ports in a mode that
> will only pass AX.25 packets beginning with a certain character, which
> is called 'block' mode.  This configuration solved the interrupt
> problem.  For Echo, with the call sign being "PACB or PECHO", only
> packets beginning with the character 'P' can get through the SCC and
> generate a CPU interrupt.
> A digi packet has the call sign of the destination station first,
> followed by the satellite (relay) call sign, and then the originator
> call sign. Therefore, the digi packet will not get through the SCC
> unless the destination station call sign begins wit   htheletter'P'.
> A great deal of effort was put into solving this problem.  This 
> solution
> is the only one at the present time that has worked well enough to
> implement in the Satellite.

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