Hi,
I think you are asking about 10/100Base-Tx ( twisted pair cable) pin outs.
We need to review how the technology evolved to understand the difference better.
For 10/100base-tx the standard itself specifies a separate pair for transmit and receive.
Whereas In earlier technologies like 10base5,10base2, which are co-ax based there is no way to implement a full-duplex link, as you dont have a separate pair of wires to do the simultaneous transmission.
So when those standards where there, the ethernet NIcs and the relevant circuitary were designed to run on only half-duplex. There is no possibility to run full-duplex.
As the later 10/100base-tx standard have specified a separate pair for transmit/receive, this opens up a possiblity to trasmit and receive simultaneously.
To accomplish the same full-duplex logic was built in to the circuitary of the 10/100Base-tx NICs.
Now the nic know that there exists a separate path for transmit and receive and it can do simulateneous data transfer on both direction. When these NIC runs on full-duplex, the Collision detection logic is disabled in the circuitary.
When the same NIcs run on half-duplex in 10/100base-tx media, they still use the separate pairs for transmit receive
Pins 1,2 used for transmit.
Pins 3,6 used for receive.
But only one action can be performed at any time. Collision detection logic is activated in the circuitary when running on half-duplex mode.
The circuitary locks the operation to happen in only one direction.
When the NIC is transmitting data, if it receives any data, it detects it as a collision.
Hope this clarifies your query.
-VJ
