3Com 3CR990-TX-95, 3CR990-TX-97, 3CR990SVR97, 3CR990SVR95 Troubleshooting Hubs, Cabling Pinouts

Troubleshooting a Network Connection 137

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.

1 2 3 4 5 6 7 8

	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	5	5
6		6	RD– 6	6 RD–
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