Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!sdd.hp.com!decwrl!sgi!rpw3@rigden.wpd.sgi.com From: rpw3@rigden.wpd.sgi.com (Rob Warnock) Newsgroups: comp.dcom.lans Subject: Re: 802.3 AUI/MAU questions Message-ID: <66113@sgi.sgi.com> Date: 4 Aug 90 09:09:38 GMT References: <9505@goofy.Apple.COM> Sender: news@sgi.sgi.com Reply-To: rpw3@sgi.com (Rob Warnock) Organization: Silicon Graphics, Inc., Mountain View, CA Lines: 35 In article <9505@goofy.Apple.COM> yarak@apple.com (Dennis Yarak) writes: +--------------- | ... Does anybody know, | practically speaking, whether a problem exists with reliability or | operation, for either the host or the MAU, when an MAU remains powered | (and therefore driving AUI signal circuits) while its AUI host is | completely shut down? Or perhaps someone can point to where the standard | covers this situation? +--------------- The Ethernet Specification (don't know about the 802.3 spec) says that the common mode voltage on the transceiver cable pairs is set by the controller (AUI), not the transceiver (MAU), and that the common-mode voltage shall be between 0 and 5 volts (ref'd to the "ground" pin in the transceiver cable). The way most transceivers handle this (and the required D.C. isolation) is to use transformers on the transceiver end of the cable. Thus, no problem. Also, a completely powered-down station's output drivers will be effectively shorted together (Vcc and Gnd both zero), and so it is unlikely [but possible, one might suppose] for the signal on the transmit pair to exceed the required threshold (a couple hundred millivolts differential) to turn on transmit in the transceiver. However... *while* powering down, it is quite likely that the station may generate garbage into the transceiver, and thus into the net. But this garbage is unlikely [one hopes!] to have a valid CRC... -Rob ----- Rob Warnock, MS-9U/510 rpw3@sgi.com rpw3@pei.com Silicon Graphics, Inc. (415)335-1673 Protocol Engines, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94039-7311