I have often wondered why the pairs on Ethernet cables don't go on consecutive connector pins.
The Ethernet 10BASE-T specification assumed that existing building telephone wiring would be used in many applications. Hence Ethernet wiring is based upon Bell System USOC (Uniform Service Order Codes). These codes applied to 4, 6 and 8 pin RJ connectors and arranged the pairs starting with the middle pins. The next pair went on the adjacent pins and so on.
So, a 4-pin RJ14 would be wired pair 1 = 2 and 3, pair 2 = 1 and 4.
A 6-pin RJ25 would be wired pair 1 = 3 and 4, pair 2 = 2 and 5, pair 3 = 1 and 6.
An 8-pin RJ61 would be wired pair 1 = 4 and 5, pair 2 = 3 and 6, pair 3 = 2 and 7, pair 4 = 1 and 8.
For RJ45 the EIA/TIA standards body modified the 8-pin RJ61 wiring to improve signal quality on the outer pairs. They kept the inner 4 signals the same but moved the outer pairs onto adjacent pins.
Hence RJ45 pairing is 1 and 2, 3 and 6, 4 and 5, 7 and 8.
The following web page describes all this in more detail: http://www.homephonewiring.com/jack-standards.html
Paul
Hi,
I am trying to connect the mbed directly to a windows box via ethernet (no swtich/hub!) for testing
I assume all I need is a crossover cable, but I am having trouble seeing any packets on the PC end (running simple http client and ethernet spam apps - my wiring is as per EMAC project)
Can someone please point out the (many) obvious things I am overlooking!
Thanks