Ethernet 1973/1974

[Paul Koning]

> On May 16, 2017, at 11:47 PM, Shoppa, Tim via cctalk <cctalk at classiccmp.org> wrote:
> 
> Al just recently put this up on Bitsavers, November 1974 drawings for the first 2.94MHz Ethernet transceiver: http://bitsavers.trailing-edge.com/pdf/xerox/alto/ethernet/Ethernet_Transceiver_Electrical_Characteristics.pdf
> 
> 
> Neat to see the 15 pin AUI to Thicknet transceiver (well, a lower bandwidth version of the 10MHz ones I grew up with) drawn out so clearly. Also shows something I've never seen in real life, an "Ethernet Dummy Transceiver" which is, I guess, something like a two-port DELNI ? (obviously showing my DEC introduction to AUI Ethernet there.)

It seems that way.  One interesting aspect in this design, if I read it correctly, is that there isn't a "collision detect" function in the transceiver the way there is in the 10 Mb D/I/X Ethernet.  Instead, the transceiver delivers "TRDATA" (received data) and "TROTHER" which is the XOR of received and transmit data.  So it looks like TROTHER != 0 means collision.  In the later Ethernet, collision is detected in the transceiver and signaled to the NIC with a signal that's stretched out as needed.  For example, in the DELNI schematics (also on Bitsavers) you can see a state machine to do that -- unlike the combinatorial logic in this "dummy transceiver".  I wonder how it was done in the 10Base5 transceivers; I have a vague memory that it's partly an analog process but that may be confusion.  (I may be mixed up with the "packet voltmeter" -- an A/D project at DEC to build a device that would make a map of a coax segment by measuring the voltages of each station's packets using sensors at both cable ends.)

	paul