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[aprssig] A TNC2 (clone) question

Stephen H. Smith wa8lmf2 at aol.com
Mon Nov 14 17:17:32 UTC 2011

On 11/14/2011 9:24 AM, kc8sfq at mei.net wrote:
> Hi  All:
> I have an old MFJ1274 TNC. I'm thinking of putting it, along with a radio and 
> a gel cell battery, in a waterproof box for use as a temp DIGI for SAR and 
> similar such field use.
> What I know about the MFJ:
> 1: It won't work with APRSISCE/32 (my fav) since it won't stay in KISS mode.
> 2: It'll require a new EPROM for extended KISS operation. That's about $40-50 
> by the time it's all up and running. There are better and easier ways to do 
> that for the same money.

Replace the eprom with the last-ever TAPR Release 1.1.9 firmware which has a 
perfectly serviceable KISS implementation in it.

> My questions:
> 1: Will the MFJ1274 support the new n-N paradigm? or will it need to be 
> addressed by it's specific callsign-SSID? (I know it's been upgraded to at 
> least 1.2.9 firmware)

Not as-is, with the exception that if all you want it to do is be a home-style 
first-hop fill-in digi, then set it's callsign "alias" to "WIDE1-1" . (The 
first-hop WIDE1-1 digipeating doesn't depend on the TNC having  the 
APRS-centric callsign-changing/substituting capability described below.)

> 2: will the MFJ even function as an APRS DIGI? (my understanding is that 
> there's nothing special about an APRS packet as far as a DIGI is concerned)

But there ARE major differences between classic connected packets and APRS 
packets !

 From the standpoint of the AX.25 protocol used by both "classic" packet and 
APRS,  the path settings are the callsign(s) of the station(s) being asked to 
relay the packet.  In classic packet, these would be the actual calls of the 
stations involved; i.e. you had to know in advance the calls of the stations 
involved to get a packet relayed.  In APRS, all stations respond to  the same 
generic "callsigns" of WIDE1-1, and/or WIDE2-n.

Current "New Paradigm" APRS depends on digipeaters being able to actually 
CHANGE these "callsigns" as packets pass through them -- something classic 
packet NEVER did.    APRS digis modify these "calls" as they pass the packet 
on, to track and control the number of relay hops.   For example,  WIDE2-2 
becomes WIDE2-1 which becomes WIDE2-0 as a packet passes through 3 successive 
digis.     This handling of callsigns is unique to APRS.

Many of the new-breed low-cost KISS-only APRS-only TNCs such as TinyTrack 4, 
Tracker2, TNC-X, AGW uTNC, etc  CAN do APRS-style digipeating stand-alone.   
The ONLY "classic" TNCs able to do this off-the-shelf stand-alone (i.e. 
unassisted by computer programs), were the Kantronics KPC3 family TNCs.   
PK-232s, TNC2s, etc require a computer program running through a KISS interface 
to do modern APRS digipeating.

You can endow almost any TNC-2 clone, such as your MFJ with this ability by 
replacing it's OEM firmare with the freeware UI-Digi firmware that converts the 
TNC2 to a dedicated APRS digipeater.  However it becomes STRICTLY a stand-alone 
digipeater, and can no longer be used for local transmit/receive by a an 
attached PC.

See the full history and underlying logic of APRS digipeating here on my website:

"APRS 101"  Explanation of APRS Path Selection & Digipeating



Stephen H. Smith    wa8lmf (at) aol.com
=== Now relocated from Pasadena, CA back to 8-land (East Lansing, MI) ===
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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