APRS Digipeatersoftware DIGI_NED
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Connecting a WX station

DIGI_NED has the ability to pass data from the serial port to RF. One of the uses can be to pass raw ASCII data strings conforming to the NMEA 0183 Version 2.0 specification to APRS or data from some weather stations.

DIGI_NED can be used transmit data from the serial line to any of the configured output ports. DIGI_NED expects serial data in the form of "sentences". These are terminated ASCII lines. DIGI_NED will pick up selected lines based on the first characters in the line. DIGI_NED will transmit at regular intervals the latest received complete line of data.

With this DIGI_NED can send data from a GPS, Ultimeter or other devices connected via a serial line. You can sent multiple sentences if you want.

DIGI_NED will automatically do a checksum-check if a checksum is present. Checksums terminate a "sentence" with "*xx" where "xx" is the hexadecimal sum. If the asterisk is found in the third position before the end of the line DIGI_NED assumes a checksum is present. The checksum is compared with an internally calculated checksum. Sentences that fail will be ignored. The calculation follows the NMEA standard used for GPS devices among others. The checksum is an EXOR operation staring at the second character up to, but not including, the "*".

If no checksum is present the data is accepted as is. An examples of a start of a sentence is:

For GPS$GPRMC or $GPGGA
For Ultimeter$ULTW or !!
For Directon finder%

If the wanted type of sentence is not specified (empty line following the "serial:" rule) all the data on the serial input is accepted (if it doesn't fail the checksum). Note that DIGI_NED will only output the most recently read complete line in this case. Normally DIGI_NED transmits a packet for every specified sentence, so if you specified "$GPRMC $GPGGA"
then both the $GPRMC and $GPGGA sentence will be transmitted.

DIGI_NED will convert any control character on the serial line into a dot ".", except which is the line terminator and which is discarded.

The interface is always 8 bit, no parity, one stop bit. Accepted speeds are 1200, 2400, 4800 and 9600 baud. This covers the multitude of serial input streams, including NMEA. The interface uses hardware flow control on send an receive. For a 3 wire connection short cut RTS and CRS and short cut DTR, DSR and DCD in the connector at the DIGI_NED side of the wire.
Software-flow control (Xon/Xoff) is not used

Example of the serial command:

serial: 10 all com3 4800 GPSODN,WIDE,WIDE3-3 $GPRMC $GPGGA

This will pass the NMEA GPRS sentences $GPRMC and $GPGGA directly to RF.
For example this is one line a GPS could send:

$GPRMC,040302.663,A,3939.7,N,10506.6,W,0.27,358.86,200804,,*1A

If you have a weather device like that sends out messages compatible with the APRS raw weather format then you can connect the device directly to the serial port. According the the APRS specification the following APRS Data Type Identifiers are used in Weather Reports containing raw data:

! Ultimeter 2000
# Peet Bros U-II
$ Ultimeter 2000
* Peet Bros U-II

Examples (also from the APRS specification):

Ultimeter 2000:
!!006B005803500000----03E9--------002105140000005D
Or
$ULTW0031003702CE0069----000086A00001----011901CC00000005

Peet Bros U-II:
#50B7500820082
Or
*7007600000000

If you have any other WX station and if it is compatible with the Ultimeter or Peet Bros devices then you can connect it directly to DIGI_NED.

In all other cases you need to convert the data from whatever your station sends to one of the known formats. If you do that then it is best to convert it to the APRS Weather report format as specified in the APRS specification because there are likely more clients can read that format instead of the Ultimeter or Peet Bros formats.

More information about the serial command can be found at 'serial command' or in de 'digi_ned.ini' file

Note: for DOS only one "serial:" rule is accepted

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