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- Path: sparky!uunet!sun-barr!ames!agate!dog.ee.lbl.gov!nosc!crash!cmkrnl!jeh
- From: jeh@cmkrnl.com
- Newsgroups: sci.electronics
- Subject: Re: Coax Question
- Message-ID: <1992Jul25.120158.600@cmkrnl.com>
- Date: 25 Jul 92 19:01:58 GMT
- References: <1992Jul20.140143.19341@yang.earlham.edu> <1992Jul22.052519.901@athena.cs.uga.edu>
- Organization: Kernel Mode Consulting, San Diego, CA
- Lines: 21
-
- In article <1992Jul22.052519.901@athena.cs.uga.edu>,
- mcovingt@athena.cs.uga.edu (Michael A. Covington) writes:
- > [very nice explanation of "reflections" due to impedance mismatches]
- > That's why we terminate Ethernet cables with resistors. It prevents
- > reflections.
-
- well, yes, and this is important when the cable is "long". But there's another
- reason. The resistors (or equivalents) must also be there with arbitrarily
- short Ethernet cables, since the "carrier" on Ethernet is merely voltage
- applied through a resistor. The 25 ohms (net) resistance to ground seen by the
- transceiver (50 ohms at each end) forms the other half of a voltage divider. If
- two transceivers try to transmit at the same time the voltage on the cable is
- higher than would be expected with just one transmitter -- and that's how
- collisions are detected.
-
- Hence, very short (a few meters) Ethernets will work just fine with a 25-ohm
- resistor at one end!
-
- --- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
- Internet: jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
- Uucp: ...{crash,eisner,uunet}!cmkrnl!jeh
-