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[aprssig] Should I develop an embedded I/F to Davis WX?

scott at opentrac.org scott at opentrac.org
Sun Aug 1 18:37:31 UTC 2004


How complicated is the Davis protocol?  Does it require true RS-232 signal
levels?

You might consider starting with the OpenTracker hardware if you're
comfortable programming in C or assembly for the HC08.  It already supports
the Dallas 1-wire weather station (beta code should be available this
weekend) and having Davis support would probably be nice since that's one
WXTrak doesn't support yet.

The OpenTracker will report battery voltage through its on-board voltage
divider - at the moment it doesn't do that in weather mode, but it'd be easy
to set up.  It's also got an on-board temperature sensor, but that could be
excluded and used as another general-purpose I/O line.

I'll be working today on a demo of the power control function to turn on and
off the radio during WX mode operation for use in a low-power solar setup.
My goal is to have the whole thing run off a 5-watt panel with a 5-minute
transmit interval.  While idle, it draws about 5 ma right now - I should be
able to reduce that, but I'm having trouble with the MCU's STOP mode.

Anyway, the hardware's pretty cheap ($29/kit), the existing code is open
source (BSD license), and it'd let you avoid using a TNC entirely since it's
already got the hardware and code to do AFSK and BPSK.  Its RS-232 output is
0-5 volts, though, so it may require some extra hardware if the weather
station demands a true bipolar signal.  Once I get the surface mount kit in
production, that'll be a more convenient way to integrate it with custom
hardware.

Check out the project website at http://n1vg.net/opentracker

Scott
N1VG




-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org]On Behalf Of J. Gary Bender, WS5N
Sent: Sunday, August 01, 2004 10:22 AM
To: aprssig at lists.tapr.org
Subject: [aprssig] Should I develop an embedded I/F to Davis WX?


I am looking for advice for putting together an APRS standalone weather
station.
 I have real estate (eventually my retirement home) about 200 miles away.  I
also have way too much time on my hands.  I am thinking about writing my own
embedded interface to handle the data translation between my weather station
and
APRS/TNC.  I am competent to do the programming, but I am not sure I won't
be
reinventing the wheel.

Equipment:
  Davis VantagePro 6150
  MFJ 8621 Data Radio
  Tasco Telereader 211 TNC (with Japanese documentation!)
  -or- Pac-Comm Tiny-2 TNC
  12 VA sealed battery
  2.9 amp 21 v solar panel.

I remember using the Telereader years ago and it seemed to work with
conventional commands, so I can probably get by without having English doc.
It
is a nice small unit with low power consumption.  I don't think either TNC
would
impact my power budget much (200-250 ma).

What I need is a cheap way to fetch and convert Davis data to APRS weather
packets.  I probably should add some additional APRS intelligence to the
MCU.  I
will include some telemetry; things like battery condition and the state of
a
few switches/alarms.  I have not monitored traffic yet, but nothing shows up
near the 87315 zip code on FindU.  However, I can hear something every few
minutes full-quieting, so I probably have a digi I can hit.

I also have two Zilog Development Kits sitting here begging to be applied to
something useful.  One for the Z8Encore! And another for the Z8Acclaim!
Either
chip family has plenty of room for a lot of features.  I am guessing that an
interface can be put together for much less than $50 in small quantities.
We
are looking at the Z8... MCU, a voltage regulator, a TTL/RS-232 level
converter
or two, plus a few support components.

Basically I want to plug-and-play the Davis to the MFJ as easily as folks
say
the Peet Bros weather stations do.  It turns out there are lots of Davis
units
on eBay, so it is worth a couple hundred bucks to do it.

My questions:
1) Has this already been done for a low power solar environment?
2) Is there a list of TNC attributes that should be implemented at startup?
Reading the APRSSIG the last couple months, I see there are some parameters
that
a well behaved APRS controller should initialize.
3) Would there be enough interest in this sort of device for me to
generalize
the code so it can adapt to other weather stations and TNCs without too much
trouble?
 --
    Gary, WS5N
    Tijeras, NM  USA





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