3Com Fast EtherLink XL PCI 10/100BASE-TX manual Troubleshooting Hubs with Crossover Cable

4-14CHAPTER 4: TROUBLESHOOTING

Using wires from opposing pairs can cause signals to

be lost. For a 100BASE-TX cable, pins #1 and #2, and #3 and #6 must be on the same twisted strand.

Troubleshooting Hubs with Crossover Cable

When there is doubt whether a hub is performing correctly, or if the impedance settings are in question, a crossover cable can help you isolate the failing component.

1Connect a file server and a client PC back to back with a crossover cable to verify that the NIC and network operating system are properly configured.

2To make a crossover cable, simply connect TD+ to RD+ and TD– to RD–.

The cable performs the crossover that is usually performed by the hub. Make sure that two twisted-pair wires are used. Figure 4-7 shows the pinouts for the crossover cable. On the straight-through cable, make sure that the wires are from the same twisted pair.

If the file server and client PC function together as a small network, then either the existing cabling or the hub is the problem.

■If there is a proper crossover, the appropriate LED comes on.

■If there is a straight-through connection, the LED does not light.

■A blinking LED indicates that there is a polarity mismatch (that is, TD+ to RD– instead of TD+ to RD+).