Date   : Wed, 27 Jun 1984 08:44:49-PDT (Wed)
From   : decvax!decwrl!flairvax!kissell@Ucb-Vax.arpa
Subject: Re: Zilog SIO as a network controller

(Get thee behind me!)

First, an appology for posting this netwide, but we are not an ARPANET
site.  Besides, it might be generally amusing.

Z80 SIO's as network controllers are pretty common.  Sytek's Localnet 
broadband LAN uses SIO's running at 128Kbits.  I believe that
Corvus' Omninet uses them also (corrections, folks?), and I know of
others more obscure ("Bestnet", for instance).  At usefully high
speeds, one needs to run in a synchronous mode, because in asynchronous
modes the SIO requires a 16*bitrate clock for character framing, whereas in
synch modes it uses a 1*bitrate clock, so for a given quality of clock
circuit one can run 16 times faster in synch mode.  Forget the bisync
mode, as it requires more processor intervention than HDLC mode.  At
speeds in excess of 500Kbits, you will probably need some kind of DMA
circuitry to move the data for you.  The SIO has only one DMA request
line per channel (it is a 2 channel device), so If you want to run
full duplex (and you might, depending on your access method), you will
have to send on one channel and recieve on the other.  No big deal, but
it makes some of the software a little bizzare.  I'm not going to dump
a monologue on possible access methods on the net, except to say that chapter
7 of Tannenbaum's "Computer Networks" is a good place to start, but by
no means an exhaustive look at the problem.

Kevin D. Kissell
Fairchild Research Center
Advanced Processor Development
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 after a number of repetitions"