Path: utzoo!attcan!uunet!husc6!mailrus!uwmcsd1!ig!agate!saturn!eshop From: eshop@saturn.ucsc.edu (Jim Warner) Newsgroups: comp.dcom.lans Subject: Re: Does anyone rent TDR's? Summary: Low cost TDR Keywords: TDR Ethernet Message-ID: <4879@saturn.ucsc.edu> Date: 17 Sep 88 17:16:56 GMT References: <425@aoa.UUCP> Reply-To: eshop@saturn.ucsc.edu (Jim Warner) Organization: University of California, Santa Cruz Lines: 55 In article <425@aoa.UUCP> mbr@aoa.UUCP (Mark Rosenthal) writes: >I would like to rent a Time Domain Reflectometer to check out our backbone >cable, and I have a few questions.... What is special about the Cabletron TDR is (1) it uses negative going pulses to probe the cable and (2) it can superimpose a steady negative voltage onto the cable to force all the stations to defer. The rational for using negative going pulses is that's what transceivers normally put on the cable so you minimize the possibility of electrically stressing your installed transceivers. I'll buy that. Cabletron brought one of their TDRs out here for a sales demo. I found that it didn't do any more than the setup I was using which cost a whole lot less money. I use a conventional pulse generator and an oscilloscope. The requirements for the pulse generator is that it be able to generate negative going pulses ~50 nS long and that it have a separate output to send to the trigger input of your oscilloscope. One pulse generator that is suitable is B&K model 3300, which sells for about $400. Put the pulse generator close to the scope and the end of the E-net cable. Run a BNC cable from the generator to a 'T' connector on the 'scope input. Another BNC cable goes from the o'scope 'T' to a BNC to N type adapter on the end of the E-net cable. Set the pulse generator to produce 0.5V negative going pulses before you attach it to the E-net cable. A good repetition rate is anywhere between 1 and 10 KHz. A third BNC cable goes from a spare output on your pulse generator to external trigger input on your oscilloscope. You'll be measuring the distance to reflections in time units. Convert to distance units by multiplying by 0.77 ft per ns. You should divide your observed times by two to convert round trip time to one-way time. The reason for a triggering the oscilloscope independent of the signal on the E-net cable is that it then won't trigger on attempts of hosts to transmit. This readily serves the same function as Cabletron's custom negative bias generator. I would prefer a packaged portable TDR unit if I had to do TDR in the field where there was no electricity -- say in a man hole in the middle of the street. But I don't climb around in man holes... I am especially fast to get out my TDR setup whenever (1) we are dealing with a new contractor and the office wants to know if it's OK to pay the bill (2) someone wants to convert their network cable to RG58 with a type N to BNC adapter (3) a network expansion is being done with cable from a different vendor than the original install. Of course, if your records are less than perfect, TDR also provides the authoritative method for determining the length of a cable segment. jim