From: Bob Nielsen (terhi.victor@logonet.com)
Date: Tue Jun 26 2001 - 20:35:39 EEST
On Tue, Jun 26, 2001 at 11:18:02AM +0300, Tomi Manninen OH2BNS wrote:
> On Mon, 25 Jun 2001, Niall Parker wrote:
>
> > On Mon, Jun 25, 2001 at 07:51:21PM -0700, Bob Nielsen wrote:
> >
> > [snip]
> >
> > > Thus, although you can communicate between the computer and the Baycom
> > > device at 300 baud, it would still communicate with the radio at 1200
> > > baud, although with a slower character rate (increased mark dead time).
> > > It would not be compatible with 300 baud HF packet operation.
> >
> > Actually, the TCM3105 (and similar units) are dumb modems, bit rate in
> > equals bit rate out. The frequency shift is 1 kHz, so the bandwidth occupied
> > will be wider than the conventional HF shift of 200 kHz, but most simple
> > PLL based demodulators should be able to decode it OK still.
> >
> > If you want to get the 200 Hz shift of a conventional HF modem, look around
> > for one of the older Exar based Bell 202 modems which can be retuned for
> > different frequencies and shifts by substituting resistors and/or a cap.
> > ... or bypass all that and use a sound card :)
>
> My old TNC2 clone (a Symek TNC2S) has a 300baud / 200Hz mode that is
> implemented with the same TCM3105 that is used with 1200baud / 1000Hz. I
> don't have the schematics of the TNC here right now but looking at the
> TCM3105 data sheet I would guess it uses the CCITT V.23 600bd mode of
> the TCM3105 chip. This has tx and rx tones of 1300/1700Hz and as the chip
> scales the tones with the xtal frequency, dividing the regular PAL color
> burst xtal by two should do the job (both shift and baud rate even if the
> latter probably only affects the internal filter widths - baud rates are
> in a baycom modem are generated in the PC).
That's interesting. I have the manual (in german) and schematic for
the TNC2S around somewhere and if I can find them, I'll pull out my
trusty old Langenscheidt's dictionary and take a look at that.
In my old age (62), I had forgotten that baycom did some unconventional
things with the serial port (which is why it works, of course).
73, Bob
-- Bob Nielsen, N7XY qzvulgiw@bcinternet.net Bainbridge Island, WA http://www.oz.net/~nielsen IOTA NA-065, USI WA-028S - To unsubscribe from this list: send the line "unsubscribe linux-hams" in the body of a message to terhi.victor@logonet.com
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