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From: tom@rsp.UUCP (Thomas Ruf)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: minimum length ethernet packet
Summary: LANCE minimum packet size
Message-ID: <374@rsp.UUCP>
Date: 17 May 89 10:11:57 GMT
References: <8905151510.AA00477@mitre.arpa>
Organization: RSP Datensysteme, W-Germany
Lines: 22

In article <8905151510.AA00477@mitre.arpa>, mckee@MITRE.MITRE.ORG (H. Craig McKee) writes:
> .....             During power-up and self-testing, many devices
> (including DECserver 100's) create a 32 byte packet (36 bytes with
> CRC).  This packet is transmitted to the network.  The device expects
> to receive the packet back from the network.  This is how the device
> determines that the network is operational. ..............
> .....
> I don't have the LANCE spec sheet, but it seems to me the chip set
> should be passive with respect to packet length; that it is the 
> responsibility of the driver routine to insure proper packet length.

The LANCE is a half duplex device, which is sufficient for normal Ethernet
operation. If you want to perform an external loopback test, as DEC
does it during power-up, you need full duplex. Therefore, the LANCE
has a special "external loopback test mode" which allows simultaneous
send and receive operation. However, due to the size of the fifo,
the packet length is limited to 36 bytes.

Thomas
-- 
Thomas Ruf				Schneider & Koch GmbH
{uunet,mcvax}!unido!rsp!tom		West-Germany