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Re: 38k4



-----Ursprüngliche Nachricht-----
Von: Matt Donohoe <TheRelic@dingoblue.net.au>
An: amsat-bb@AMSAT.Org <amsat-bb@AMSAT.Org>
Datum: Donnerstag, 25. Mai 2000 12:22
Betreff: [amsat-bb] 38k4


 >I have become interested in 38k4 data...

 >Anyway, I am interested in a "homebrew" type
 >of TNC/Modem that can accomodate
 >the 38k4 stream from satellites such as UO-36...

Hi dr OM Matt!

the problem is, that the Z80 COU based TNC2 are
normally not able to decode signals above 19200
baud. The CPU is interrupted after every byte and
an interrupt rate of say 5 kHz is too much for a
4.9 MHz clocked 8 bit CPU. With faster Z80 (20 MHz),
a maximum of 38kbit/s may be achieved in KISS mode.

The only way to decode AX.25 faster is to use other
processor platforms and additional hardware to buffer
the fast ingoing and outgoing data streams.

But: other processor means to say good bye to the
proven Z80 TNC2 software. Alternative AX.25 software is
available for the PC (486/Pentium etc.) and for the
Motorola MC68302 CPU.

The MC68302 is a special communications processor which
is able to receive (and transmit) up to 8 complete data
packets and store it via DMA without any participation
of the CPU kernel. The HDLC decoder is done by a RISC-
hardware on chip. So, you can receiver and decode up to
1.6 Mbit/s simultaneously with this chip, makeing it the
ideal choice for packet radio.

The TNC3 type uses this CPU and there exists a lot of
amateur packet radio firmware with it (WA8DED-hostmode,
KISS, SLIP, Hayes, 6pack etc.).

The German Baycom group http://www.baycom.org has
developed some low cost modems which buffer AX.25 and
process the raw data with a PC. As far as I know, this
'EPP-Adapter' can handle up to 300 kbit/s.
The TNC3 (2 modems, 1.6 Mbit/s system speed) is available
from SYMEK (http://symek.com) in Germany.


73! Ulf, DK9SJ

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