Allied Telesis CentreCOM AT-3675 Bridge User Manual page 24

Data Cabling Techniques
Figure 16: Hub-to-MAU
Wiring
A.Usable and
B. Unusable
22
RJ45 Pin
1
TD+
2
TD-
3
RD+
4
Not Used
5
Not Used
6
RD-
7
Not Used
8
Not Used
This cable will not work for 10BASE-T.
Note that 3 and 6 are twisted, but are not a pair.
RJ45 Pin
1
TD+
2
TD-
3
RD+
4
Not Used
5
Not Used
6
RD-
7
Not Used
8
Not Used
Diagram A is correct because the proper pairs are twisted together.
Diagram B is incorrect because the wires for the receive pair, pins 3 and 6, are
not twisted together. This could result in excessive common mode noise and
an unacceptably high data error rate.
If you pair pins incorrectly, for example as shown in Figure 16, Diagram B,
your network may have a high data error rate. In a straight-through cable,
the transmit pins, 1 and 2, are paired, as are the receive pins, 3 and 6. In a
cross-over cable, shown in Figure 17, pins 1 and 2, TD+ and TD-, are paired,
as are pins 3 and 6, RD+ and RD-. When pins 1 and 2 are crossed over, they
connect to pins 3 and 6 respectively, and pins 3 and 6, when crossed over,
connect to pins 1 and 2, respectively.
A
1
Pair twisted as per Level 3, 4 or 5 cable.
1
1
B
RJ45 Pin
1
TD+
2
TD-
3
RD+
4
Not Used
5
Not Used
6
RD-
7
Not Used
8
Not Used
RJ45 Pin
1
TD+
2
TD-
3
RD+
4
Not Used
5
Not Used
6
RD-
7
Not Used
8
Not Used