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[aprssig] The ideal APRS digi

Henk de Groot henk.de.groot at hetnet.nl
Mon Sep 27 19:04:13 UTC 2004

At 20:02 26-9-2004 -0400, John Kraus wrote:
>This could also be a prime use for Digi_Ned.  If I am correct Digi_Ned can 
>receive on multiple ports and cross connect the ports and packets while 
>doing n-N. It may even be possible to have the packet identified by port 
>on the output.

Yes, sure, it can handle up to 8 ports, although in real life more than 2 
is an exception. Our local digi has 2 ports, one on 2 meter and one on 70 cm.

The easiest to make a "read-only" port is to use the default rules and 
switch off the PTT for the port that only listens. Another alternative is 
to change all the digipeating rules specifying port "1" as output there the 
digipeated packets should go to (instead of "all" which sends the 
digipeated packets to all ports). Note that such a configuration will allow 
paths like WIDE3-3 on the read only port, they are digipeated on the main 
QRG as WIDE3-2 (so excacly the same as if it was received on the main QRG).

There is one cavat, there in no DCD cross connect. You can drive 2 KISS 
modems (among other configurations), but DCD detection by the connected PC 
is through the reception of KISS data, not realy the actual carrier status 
of the RF channel. For transmission, the PC can send data to the KISS TNC 
but has no control when the packet actually goes on air (the TNC desides 
that). A KISS driver can do a best effort to do DCD checking, but only a 
hardware solution will work fully!

For Linux DIGI_NED depends on the AX.25 kernel support, I don't know if 
there is any DCD cross connect possible. Of course a hardware solution will 
always work!

I wonder how much packets a cross connected DCD will save however. The APRS 
channel should not be loaded more than 18%. This means that worst case the 
half of it (9%) could come from the DIGI (the other half is the reception 
of the original packet). So out of 60 seconds the DIGI will transmit only 
at most about 6 seconds). So a station on the alternate channel has a fair 
chance to get trough and not have too many packets jammed by the 
digipeater's own transmissions.

Maybe for the next DIGI_NED floppy version I should allow you to declare 
ports to be read-only in the configuration program to suit this type of 
application out of the box. With todays version you have to do some manual 
editing of the digi_ned.ini file. The easist is to add a "command:" rule 
with the !ptt command:

command: !ptt x0xxxxxx

This switches off the PTT for port 2, making port 2 read-only.

Kind regards,


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