Date: Thu, 24 Aug 1995 14:50:26 -0500 (CDT) From: Joe Greco <jgreco@brasil.moneng.mei.com> To: rgrimes@gndrsh.aac.dev.com (Rodney W. Grimes) Cc: fenner@parc.xerox.com, terryl@cs.stanford.edu, freebsd-hackers@freebsd.org Subject: Re: ISDN Anyone? Message-ID: <199508241950.OAA20237@brasil.moneng.mei.com> In-Reply-To: <199508241906.MAA08345@gndrsh.aac.dev.com> from "Rodney W. Grimes" at Aug 24, 95 12:06:05 pm
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Hi Rod, > 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. This is something to consider. However I am talking out of a NS data book and the only references I have seen are: NS16450/16C450/ 3.1 MHz max, 56k "top" baud rate suggested INS8250A/ INS82C50A INS8250/ 3.1 MHz max, 56k "top" baud rate suggested INS8250-B NS16550AF 8.0 MHz max, 256k "top" baud rate suggested NS16C451 24.0 MHz max, no suggested top :-) NS16C551 24.0 MHz max, 1.5M "top" baud rate suggested Now if I am reading this right the INS8250-B is a slower speed part, judging from the electrical characteristics. I do not see any lower-speed offerings on the other parts. Although it IS funny that the 16C451 can have a 24 MHz clock and they didn't bother to list a top rate, I can imagine that you could get pretty busy trying to shove a million bits per second through a device without any real FIFO. > 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. Or if you are only going 2 feet like I usually do. :-) Anyways the moral of the story is: don't try to hot rod a non-FIFO chip or you may torch the sucker. (heck, you may even torch a FIFO chip, there are no guarantees... if you can't afford to lose the UART don't play with fire). And of course if you're using non-NS parts (i.e. Startech, etc) you might want to check the specs. Inferior chips made for the PC market and all... I've run Startech at 2x without problems. ... Joe ------------------------------------------------------------------------------- Joe Greco - Systems Administrator jgreco@ns.sol.net Solaria Public Access UNIX - Milwaukee, WI 414/342-4847
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