Date: Thu, 24 Aug 1995 12:06:05 -0700 (PDT) From: "Rodney W. Grimes" <rgrimes@gndrsh.aac.dev.com> To: jgreco@brasil.moneng.mei.com (Joe Greco) Cc: fenner@parc.xerox.com, terryl@cs.stanford.edu, freebsd-hackers@freebsd.org Subject: Re: ISDN Anyone? Message-ID: <199508241906.MAA08345@gndrsh.aac.dev.com> In-Reply-To: <199508241644.LAA19807@brasil.moneng.mei.com> from "Joe Greco" at Aug 24, 95 11:44:55 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
^^^^^^^^^
Carefull here, many Uarts are only rated for on f(max) of 5Mhz, others
are good to 8Mhz. I think all 16550AFN's are rated for 8MHz, but not
sure if they still had the -5/-8 speed option that late in the game.
> 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).
Watch the capacitance on that shielded cable, the higher the pf/foot the
shorter it needs to be. If you use high quality, low loss, low capacitance
individually shielded twisted pair data cable grounded one side of each
pair for the TX and RX (yea, okay, so you need a few more wires :-) you can
run 230KB quite a distance. A scope comes in pretty handy to see your
signal quality and the receiver as well.
>
> 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). :-)
:-).
--
Rod Grimes rgrimes@gndrsh.aac.dev.com
Accurate Automation Company Reliable computers for FreeBSD
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