3Com 3CR990 manual Connect a straight-through cable from the PC to the hub

64CHAPTER 9: TROUBLESHOOTING

2Connect a straight-through cable from the PC to the hub.

The hub performs an internal crossover so that the signal can go from TD+ to RD+ and TD– to RD–. When you look at an RJ-45 connector from the front (that is, the opposite side from where the wires enter the connector), pin 1 is identified on the right side when the metal contacts are facing up.

3Make sure that the TD+ and TD– wires are twisted together, and that the RD+ and RD– are twisted together.

Using wires from opposing pairs can cause signals to be lost.

Troubleshooting Hubs A crossover cable can be used to identify the type of failure when hub

performance or connectivity is in question.

To use a crossover cable:

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, connect TD+ to RD+ and TD– to RD–.

The cable performs the crossover that is usually performed by the hub.

Cabling Pinouts The following illustration compares the cabling pinouts for straight-through and crossover cables.

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	Straight-through						Crossover
	10BASE-T cable						10BASE-T cable
1		1	TD+ 1				1	TD+
2		2	TD		2		2	TD
3		3	RD+ 3				3	RD+
4		4		4			4
5		5	RD	5			5	RD
6		6				6	6
7		7		7			7
8		8		8			8

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

When a crossover cable is used, the LED on the NIC functions differently than it would under normal operating conditions. For example, with a correct crossover connection, the LED lights, whereas with a straight-through connection, the LED does not light. If you make a crossover cable and the polarity is mismatched (that is, TD+ to RD– instead of TD+ to RD+), the LED blinks.