[aprssig] Short paths in APRS packets

Matti Aarnio oh2mqk at sral.fi
Tue Oct 27 12:20:40 EDT 2009

On Tue, Oct 27, 2009 at 08:46:24AM -0700, Greg Clark wrote:
> I have two questions regarding building up AX.25 packets.
> 1) If the "path" fields are shorter than 7 characters, what do they
> get padded with. Spaces?  Nulls?  Most "common" paths don't have this
> problem, since WIDE3-3 is seven characters.  I'm trying to configure
> my tracker for use with the ISS using the paths ARISS and SGATE, and
> I'm not sure what to put in the two remaining bytes when I build the
> packets.  I'm asuming this field is fixed in length at 7 characters.

"WIDE3" is 5 characters, the hyphen and following number are denoting
value of SSID bits in the 7th byte of the address field.

The field of callsign characters is only 6 bytes long, so you will need
to pad it - with spaces (0x20).

 > 2) What about the '-' in the path WIDE3-3, is it really a 'dash' in
> the packet? or is it a "space".  I'm pretty sure that at this point in
> time, I'm sending a "space", but not sure if that's the correct way to
> build the packet.  It does, however, seem to be working properly as
> is.

The "-" is not part of callsign at all.

Both "space" (0x20) and NUL (0x00) appear in encodings on air,
but official one is "space" (0x20).
> Yes, I am aware that everything needs to be shifted left by one  bit,
> and the last entry in the string of paths has the least significant
> bit set.

However you did not quite get that 7th byte is a collection of bits
and bit-fields.

Have you downloaded the AX.25 protocol specification from TAPR web?
I recommend version 2.0,  _not_ 2.2, when working on APRS.

> Thanks,
> Greg K7RKT

Hopefully this helps,

73 de Matti, OH2MQK

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