Path: utzoo!mnetor!uunet!mcvax!jack
From: jack@cwi.nl (Jack Jansen)
Newsgroups: comp.sys.atari.st
Subject: Re: atari to appletalk quest
Message-ID: <253@piring.cwi.nl>
Date: 28 Mar 88 13:51:03 GMT
References: <8803071434.AA04788@ucbvax.Berkeley.EDU> <512@uhnix2.UUCP> <85@obie.UUCP>
Organization: AMOEBA project, CWI, Amsterdam
Lines: 42

In article <85@obie.UUCP> wes@obie.UUCP (Barnacle Wes) writes:
>In article <512@uhnix2.UUCP>, uace0@uhnix2.UUCP (Michael B. Vederman) writes:
>> Unfortunately, this cannot be done with the ST as it stands without external
>> hardware (probably thru the DMA port).  The reason for this is that the
>> AppleTalk network uses a baud rate far greater than the ST can produce.
>
>Not true.  The MIDI port can be easily driven at 500K bps, much faster
>than Apple's LocalTalk hardware, which is essentially an RS-422.  If
>....
This is not the only problem. The main problem is that the UART used
in the apple (don't remember the number. I seem to recollect that
it's a Zilog chip) is a very nifty little device. Among other features,
it can be programmed to generate NRZ output in stead of straight
binary.
[NRZ is an encoding scheme where a "1" bit is encoded as 1 transition,
and a "0" bit as 2 transitions, like this:
Data    0   0   1   0   1   1
Normal _________----____--------
NRZ    _--__--__----__--____----
]
This is crucial for appletalk: the appletalk connector box is merely
a bunch of transformers wired as a fork. This has two advantages:
- You don't hear yourself talking.
- All machines are completely galvanically separate from the cable,
  but you don't have to power the cable (as you would have to do in
  case of using optocouplers.

Now, using transformers makes it next-to-impossible to use normal
binary: you would never be able to reconstruct the signal after it
has passed through two transformers: the phase-shift would differ per bit.
No such problems with NRZ encoding, however: the only frequencies
used are F and 2F (if you send at F baud), so a low-pass filter will
convert you signal to almost-sine-waves, which can be turned into
squares again at the receiving side.

Thinking up the hardware to convert normal binary to NRZ is left
as an exercise to the reader.
Oh yes, and tell me if you find a scheme that'll cost less than, say,
$10 and can be powered from the MIDI port:-(
-- 
	Jack Jansen, jack@cwi.nl (or jack@mcvax.uucp)
	The shell is my oyster.