Create a Loopback Plug for an RJ-45 Ethernet Interface
To create a loopback plug, cross pin 1 (TX+) and pin 3 (RX+) together, and cross pin 2 (TX-) and pin 6 (RX-) together. You need the following equipment to create the loopback:
A 6-inch long CAT5 cable
An RJ-45 connector
A crimping tool
Figure 1 illustrates how to create a loopback plug for an RJ-45 Ethernet interface.
When you create and then test a physical loopback, you are testing the RJ-45 interface of the PIC. This action is recommended if a field engineer is available to create the physical loop as it provides a more complete test of the PIC.