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Re: P3D G3RUH 9600 telemetry decoding



> To: amsat-bb@AMSAT.Org
> Subject: Re: [amsat-bb] P3D G3RUH 9600 telemetry decoding
> 
> From: "Howard Long" <howard@hanlincrest.co.uk>
> Subject: [amsat-bb] P3D G3RUH 9600 telemetry decoding
> Date: Tue, 26 Sep 2000 23:30:05 +0100
> 	[...]
> I'm aware that during the initial post-launch period, P3D will be sending
> 9600bps telemetry back to earth. Now it's not going to be packet, just a
> pure data stream.
> 
> The theory, as I understood it from James G3RUH, was that we hook up the
> 9600 modem portion of our TNCs to the serial port of a computer and capture
> as much telemetry as we can in the capture mode of a terminal emulator.
> 	[...]
> Now my question is is am I right in thinking I can simply hook up a MAX-232
> circuit to the modem header and plug it into my computer? If that's the case
> things will be rather simple. Can anyone tell me if I'm missing anything?

I suspect that the data stream from P3D will be synchronous data
(one bit after another with no start and stop bits for each character,
similar to packet), while your serial port will be expecting
asynchronous data (a start bit at the beginning of each character,
a stop bit at the end of each character, and no timing relationship
between characters).

I suspect that things would work better if your computer had a card
that handles synchronous data or a software solution similar to what
works with Baycom-styld modems.

I assume (or, rather hope) that the telemetry data stream has a flag
now and then so that the receiver can figure out how the bits are
aligned into bytes.  I assume (again) that the telemetry will be sent
as packets encapsulated within an HDLC-like frame, but won't contain
AX.25 headers.

But this is all a guess on my part.

-tjs

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