Re: Ethernet card- alternatives

[dempson@swell.actrix.gen.nz (David Empson)]

In article <1993Dec7.192606.9995@porthos.cc.bellcore.com> 
ccw@ctt.bellcore.com (Christopher Wood) writes:
> In article <9311161706.AA02356@fert2.fe.psu.edu>,
> jaj4922@fert2.fe.psu.edu (Jason A. Jeffries) writes:
> 
> |> That's strange; if our ports can really go 233k, I wonder why we can't use
> |> them as serial ports that fast...
> 
> My recollection is a bit fuzzy; I've never studied a schematic, or
> looked at the code.  Something about external clocking, and
> synchronous vs. asynchronous timing.  When used synchronously, the
> port reads in a byte, copies it into a buffer, and gets the next one,
> without any interrupts between bytes. 

No external clocking is involved, and it has nothing to do with the
byte coding methods.

AppleTalk uses SDLC (bit-synchronous) with self-clocking data
(frequency modulated).  The transmit clock is generated internally
using the fastest available clock speed (230400 bps) off the SCC's
digital phase-locked loop circuit.  The receive clock is derived from
the same circuit, with feedback from the receive data to handle
bit-synchronisation.

Asynchronous mode is used when you are running the port as a standard
serial port.  Receive data is sampled at a higher rate than with
synchronous, to detect the edges in the data.  This causes a "divide
by two" effect, resulting in a maximum data clock rate of 115200 bps.

If the baud rate divider is used, the highest baud rate that can be
set is 57600 (a divisor of two), then 38400, 28800, 23040, 19200, etc.
I haven't tried bypassing the baud rate generator (it requires
reprogramming some of the SCC registers), but if this works, then you
could do 115200 bps, but can't get faster than this in asynchronous
mode without using a faster clock.


The "handshake in" line on the serial port is able to provide a clock
signal to the SCC.  I'm only aware of one device which uses this: the
Apple MIDI interface.  MIDI uses a non-standard bit rate, which the
SCC cannot generate internally (31250 bps, I think), so it is provided
as an external signal by the MIDI interface.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand