> On May 16, 2017, at 11:47 PM, Shoppa, Tim via cctalk <[email protected]>
> 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
