Date: Thu, 24 Aug 1995 11:44:55 -0500 (CDT) From: Joe Greco <jgreco@brasil.moneng.mei.com> To: fenner@parc.xerox.com (Bill Fenner) Cc: terryl@cs.stanford.edu, freebsd-hackers@freebsd.org Subject: Re: ISDN Anyone? Message-ID: <199508241644.LAA19807@brasil.moneng.mei.com> In-Reply-To: <95Aug24.092440pdt.177475@crevenia.parc.xerox.com> from "Bill Fenner" at Aug 24, 95 09:24:38 am
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> Er, forgive me if I'm being obtuse, but don't you need a higher serial rate
> than 115200 to get full performance out of 2 B channels?
Yes. You get about 90% cap at 115200.
However, I challenge you to plug a TA into a standard PC serial port and go
faster than that. :-) (The UTA-220 can't hit 230.4K at this point in time
anyways, so the picture *was* the optimal setup, and a PC serial port can't
go faster with a standard crystal).
For the brave, you can take a standard PC serial card and play
switchum-crystalum. I personally prefer and special-order STB's DSP-550
cards (2 9-pin serial and a bidir parallel, Startech 16552 part) which lend
themselves to such a modification quite nicely, as they use a standard
1.8432MHz crystal. Some motherboards with built in serial and/or all-in-one
multifunction cards apparently derive the clock from a higher frequency
crystal and may need those rates for other things (not like I'd feel too
comfortable replacing crystals on a MB).
The surgery is simple. Pull the 1.8432MHz crystal. Insert a 3.6864MHz
crystal, or a 7.3728MHz crystal. The former is 2x, the latter is 4x. Now
when you talk to that port at 9600 baud you are actually at {19200, 38400
depending on 2x or 4x}. Or if you talk to that port at 115200 you are
actually at {230400, 460800}. You lose the ability to do some of the lower
speeds, but then again... so what.
Note: Chintzy line driver chips may not cope too well. Note^2: The faster
you go, the shorter and more shielded your cable should be!!! (I haven't
had any problems at 230400 but then I tend to build quality cables myself).
For the chickenhearted: I have a pair of DSP-550's modified to run 230400
that I no longer use (I am tending to purchase MB's with built in serial
ports). If anybody really needs one, well... hey they can get you a
20K/sec network link and they're cheaper than Ethernet cards (and you get
TWO interfaces). :-)
... Joe
-------------------------------------------------------------------------------
Joe Greco - Systems Administrator jgreco@ns.sol.net
Solaria Public Access UNIX - Milwaukee, WI 414/342-4847
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