Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: comp.dcom.modems Subject: Re: What is Max speed possible on phone lines? Message-ID: <2193@kitty.UUCP> Date: Tue, 3-Nov-87 10:24:54 EST Article-I.D.: kitty.2193 Posted: Tue Nov 3 10:24:54 1987 Date-Received: Sat, 7-Nov-87 04:04:59 EST References: <16064@topaz.rutgers.edu> <1070@saturn.ucsc.edu> Organization: Recognition Research Corp., Clarence, NY Lines: 28 Summary: Passing DC through a coax --> twisted pair adapters In article <1070@saturn.ucsc.edu>, eshop@saturn.ucsc.edu (Jim Warner) writes: > Questions of legality not withstanding, could someone explain how these > things work? I thought Ethernet required low frequency response in > order for collision detection to work. A collision is declared when > the average voltage on the cable gets too negative. How do you get > DC through a balun? The balun products on the market which appear to work for Ethernet employ transformers with split primary and secondary windings about a toroid core (i.e., a total of four windings). Add a bit of resistor and capacitor madness, and you have something which not only attempts to match single-ended coaxial cable to balanced pair, but which also passes DC. In the case of Ethernet, these devices are extremely prone to ground-loop current problems. I never implied these baluns were a marvel of electrical engineering; they are a Kluge with a capital K. But they do work, more or less, over short distances. *I* would never use one of these gadgets, but one of our more "enterprising" clients took it upon themselves to use 'em because their plant engineer was too lazy to fish coaxial cable through floor duct; since there was several hundred feet involved (which is no longer a "short" distance for this balun business), the results were, as expected, disastrous. If my memory is correct, these baluns first started out for short length 327X applications, and then people became bolder and tried Ethernet. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"