3Com EtherLink 3C590 User Manual page 48

Miscellaneous Checks 4-11
If you are experiencing any problems, first make sure that
your hub complies with the 10BASE-T specifications and then
check the LED. The meaning of LED activity is given below:
If the LED is on, the link beat is working.
If the LED is off, the link beat has not been established or
there is a problem with the connection between the adapter
and the hub.
If the LED is blinking, the cable polarity is reversed.