Xref: utzoo comp.dcom.modems:4597 sci.electronics:8038
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!ucsfcgl!cca.ucsf.edu!jst
From: jst@cca.ucsf.edu (Joe Stong)
Newsgroups: comp.dcom.modems,sci.electronics
Subject: Re: SW56/ISDN for Macintoshes??
Keywords: 56KB, ISDN
Message-ID: <2439@ucsfcca.ucsf.edu>
Date: 30 Sep 89 07:06:13 GMT
References: <1989Sep29.114152.4344@ux1.cso.uiuc.edu>
Reply-To: jst@cca.ucsf.edu.UUCP (Joe Stong)
Organization: Computer Center, UCSF
Lines: 31

pasek@c10sd3.StPaul.NCR.COM (M. A. Pasek) and garyf@mehlville.ncsa.uiuc.EDU
(Gary Faulkner) are both interested in 56K baud and/or ISDN communication 
for the MacII or the Mac SE.  

This has my curiosity going.  I suspect the above named folks above want a
packaged solution, however.

It was my understanding that appletalk used the regular serial ports on
ANY macintosh to do 256K baud transfers.  The serial port chips are easily
capable of 56 or 128 K(ISDN) baud, though I'm curious whether they are
"settable" to exactly 56K baud using the internal clocks.  The mac is
capable of getting external clocking, possibly with some minor change
(the people who do FastTalk AppleTalk do this)

 All you may need is a cable if you're using a 56K baud modem that can
supply a clock signal.

ISDN would probably be harder, since the stream would have to be deformatted.
The D and B chanels all scrambled on the same wires, and you'd have to
pick out the bits that belonged to you, it'd have to be pretty fast
software.  Hardware, albeit simple hardware might help to sort one of
the B channels and the D channel to the two serial ports.

My understanding is that 8250's with the standard crystal that they use
in IBM PC's can only hit 57.6K baud, and not 56K baud, that it's a 
slight mismatch in speeds.  Is this true?  Does the same problem exist
with the serial ports on a mac?  Do the mac chips have any FIFO?

Can the Mac serial ports be read via DMA?  

Will someone with more solid knowledge of this clarify, please?