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From: mikel@berlioz.nsc.com (Michael G. Lohmeyer)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet "heartbeat"
Message-ID: <1991May16.004523.21301@berlioz.nsc.com>
Date: 16 May 91 00:45:23 GMT
References: <12164@uwm.edu>
Sender: news@berlioz.nsc.com
Organization: National Semiconductor Corporation
Lines: 38

In article <12164@uwm.edu> hardiman@csd4.csd.uwm.edu (Paul V Hardiman) writes:
>While looking through vendor literature for Ethernet repeaters and
>transceivers I've noticed several references to something called a
>"heartbeat".  Can anyone give me a short summary of what a "heartbeat"
>is and whether or not I should choose equipment that supports it?
>
>Thanks,
>Paul Hardiman
>University of Wisconsin - Milwaukee

     Heartbeat is a pulse sent from the transceiver to the transmitting 
ethernet card after a packet has been transmitted.  The pulse is given
on the collision line of the AUI cable.  When transmitting an ethernet 
packet, the data is transmitted on the TX+/- line of the AUI, and the
transceiver loops that data back on the RX+/- line so that the ethernet
board can self-monitor its own transmission and do a CRC or other check
on the packet.  This allows the transmitting node to make sure that it
is transmitting ok.  In this process, the collision lines, CD+/- are not 
actually check because not data/pulses are received on these lines during
a successful (non-collision) transmission.  The Heartbeat has this effect.
After transmitting a packets, if the transmitting node does not get a 
Heartbeat signal from the transceiver, then it knows that the CD+/- lines
were not connected correctly.  If these lines were not connected correctly
during transmission, then there may have been a collision during the 
tranmission that the ethernet board did not sense.  Hence, the Heartbeat
verifies that the AUI cable is correctly connected to the board.  

     Most equipment supports it.  It is a spec for IEEE 802.3.  Generally,
you want to make sure that the transceivers and MUXes that you get 
do support it, and also allow you to turn it off.  Sometimes you need
to be able to turn it off when connecting a MUX to a repeater for example,
depending on the equipment you buy.

Mike
-------------
Mike Lohmeyer				mikel@berlioz.nsc.com
National Semiconductor Corporation
(408) 721-8075