Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!samsung!think!ames!apple!motcsd!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Avoiding a transceiver
Message-ID: <2230036@hprnd.HP.COM>
Date: 1 Dec 89 17:14:32 GMT
References: <7014@ficc.uu.net>
Organization: HP Roseville Networks Division
Lines: 39

> In article <7014@ficc.uu.net> kunkee@ficc.uu.net (randy kunkee XNX MGR) writes:
> >Question:  If you only want to connect two systems together, via Ethernet,
> >it seems that it should be possible to do this simply with a transceiver
> >cable properly adapted to rearrange the signals.  We have some special
> >situations in which this would be desireable (ie. much cheaper).  Does
> >anybody sell such an adaptor?
> 
> This is not impossible.  It is the AUI equalvilent of a cross DTE cable. 
> You don't have to worry about losing collision detection because
> each machine xmits directly into the reciever of the other machine.

In normal CSMA/CD operation, it is not necessary to have full duplex
operation of the MAC.  That is, the DTE does not have to be capable of
simultaneous receive and transmit.  Most DTEs aren't and wiring this
way would cause dropped packets.

There are other problems.  In normal CSMA/CD operation, the transmitted
signal appears on the receive pair (loopback).  Many DTEs check for this
either at power-on with a loopback test or every time a packet is
transmitted.  They will sense loss of loopback as an error condition and
may stop transmitting in response.

Also, the transceiver is suppose to activate the collision signal after
each transmission (SQE test).  If the SQE test message is not received,
some DTEs will stop transmitting.  (Since early Ethernet implementations
did not do SQE tests, some DTEs will ignore the lack of SQE test or will
only report it as an error.)
> 
> I am not sure exactally how to wire it up, but it is not impossible.
> Happy Searching...

You can not eliminate a transceiver with a passive cable, it takes some
active components. 
> 
> Tom Dixon
> AT&T Paradyne, Largo, FL
> uunet!pdn!dixon
> ----------
Pat Thaler