Re: Internal Modem

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

Supertimer <supertimer@aol.com> wrote:

> fernande@internet1.net wrote:
> 
> >Internal modems have there own UART chips which is of a variety that is fast
> >enough for whatever speed the modem is designed.  If an external modem is
> >used, the UART chip in the computer is used.  It might not (usually) be fast
> >enough to handle the modem speed.  Also, the space thing.  Plus, externals
> >cost more thank internals.
> 
> The IIgs uses the same UART as all but the newest Macintosh models.
> It goes up to 56k for connection to modems and approximately 460k
> baud (according to David Empson) for AppleTalk.

Yes, but watch out - "baud" is not very useful in its proper meaning,
unless you are interested in analog line conditions.  The most useful
term from the computer user's point of view is usually "bits per second"
(bps), which is NOT the same as "baud".  (An example: a 2400 bps modem
actually communicates at 600 baud, but has four data bits per baud.)

The Zilog SCC (Serial Communications Controller) used in the IIgs
supports asynchronous comms (like with a UART) up to 57600 bps.  It
might also support 230400 bps by reconfiguring some option bits, but the
IIgs cannot really cope with this.

The SCC also supports synchronous comms, in which a clock signal
(possibly encoded in or derived from the data) is used to determine bit
positioning, instead of using start and stop bits.  The SCC supports
rates up to 230400 bps using the built-in capabilities, and can also
accept an external clock for arbitrary data rates (possibly much higher,
though this wouldn't be useful in the IIgs).

Incidentally, the term "UART" is not appropriate for the SCC, because
the SCC is more than a "Univerval Asynchronous Receiver/Transmitter".
The term USART is more appropriate ("U... Sync/Aync R.../T...").

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand