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[aprssig] Modem tones, the KPC3+, and Bell 202

Phil N6TCT phil_aprssig at lapsley.org
Wed Aug 11 14:50:22 UTC 2010

Stephen H. Smith wrote:
> You can't just use a 202 (or even a V.23) modem as a replacement for a
> KPC3+.   A TNC is far more than just a modem.  It's a "PAD" (Packet
> assembler/disassembler) that buffers a block of data to send, and then
> adds sending and destination callsigns, routing information,
> error-detection checksums, etc to the payload data.    A regular
> "dumb" 202 modem would require a considerable amount of software on
> the host computer to emulate the functions of a TNC.
Sorry, let me rephrase my question.  I wasn't suggesting using an actual
Bell 202 modem vs. a V.23 modem; I understand the difference between a
modem and a TNC.  What I was asking was really two questions:

(1) is the underlying modulation scheme used in AX.25 Bell 202 (i.e.,
1200/2200) or V.23 (i.e., 1300/2300)?  I have always understood it to be
Bell 202.

(2) if the answer to question 1 is indeed Bell 202, then why would a
Kantronics KPC3+ be using V.23, and is this likely to cause
interoperability problems, or is the 100 Hz difference in AFSK tones
"close enough"?


Phil, N6TCT


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