[GreenKeys] AMTOR/SITOR/NAVTXT Protocol

[hwhall at compuserve.com]

>
last night finished a new memory board for my Altair 680b, an M6800
based machine from 1976 ;-)
>

Wonder what you might be using your 680b for? I have one that I've not fired up in a very long time. I'd made some mods back then to speed up the clock and work with Smoke Signal Broadcasting Company SS-50 bus products. They seemed to be a near match to the 680b buss and had dual floppy drive controller & RAM boards when Altair had apparently left the 680b behind.

Wayne
WB4OGM

 

-----Original Message-----
From: Chris Elmquist <chrise at pobox.com>
To: tony.podrasky <tony.podrasky at gmail.com>
Cc: Green Keys <greenkeys at mailman.qth.net>
Sent: Mon, Dec 8, 2014 9:12 am
Subject: Re: [GreenKeys] AMTOR/SITOR/NAVTXT Protocol


On Sunday (12/07/2014 at 08:44PM -0800), tony.podrasky wrote:
> GM OMs;
> 
> Now that WLO is transmitting in both ITA#2 and SITOR-B, I'd
> like to write a "C" program to decode the SITOR-B mode.
> 
> The research I have done tells me that it is transmitted
> at 100 Baud and uses 7-bit code.
> 
> Is that 7-bit code synchronous - or is it using a STOP bit?
> 
> I'm guessing that since a synchronous transmission over the
> air would be impractical, that they must have at least a
> single stop bit.
> 
> Does anyone know?
> 
> Does anyone know where I can go to do more research?

There are some more details in Wikipedia,

http://en.wikipedia.org/wiki/SITOR

which claims that amateur AMTOR and SITOR are the same.  So, AMTOR,

http://en.wikipedia.org/wiki/AMTOR

then reveals,

	The AMTOR protocol utilizes a 7-bit code for each character,
	with each code-word having four mark and three space bits. If
	the received code does not match a four-to-three (4:3) ratio,
	the receiver assumes an error has occurred. In error detection
	mode, the code word will be dropped; in error correction mode,
	the receiver requests that the original data be resent. AMTOR
	also supports FEC in which simple bit-errors can be corrected.

So, I interpret that to mean that there are no start or stop bits
for each character but instead the 4:3 ratio is used to decern
valid framed characters.

The SITOR-B writeup also mentions,

	SITOR has an aurally easy to identify idling
	pattern. Synchronization bursts are transmitted every second
	or so and last for slightly more than one second. Every few
	sync bursts, a Morse identifier of three letters is transmitted
	(for example NMO in Honolulu).

And so I think it is actually synchronous, with these periodic bursts
being used to resync it.

I have not played with these modes myself but after reading the AMTOR
description,

	AMTOR was developed in 1978 by Peter Martinez, G3PLX, with the
	first contact taking place in September 1978 with G3YYD on the
	2m Amateur band. It was developed on homemade Motorola 6800-based
	microcomputers in assembler code.

I'm all over that!  I have been a Motorola 6800 fan since 1975.  I just
last night finished a new memory board for my Altair 680b, an M6800
based machine from 1976 ;-)

> MicroSoft error messages written in Haiku:
> 
>         Yesterday it worked.
>       Today it is not working.
>         Windows is like that.

Amen.

-- 
Chris Elmquist

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