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207344-B B-3
The switch module receives data on pins 1 and 2 and transmits its data on pins
3 and 6. The host module ports implement this MDI standard crossover internally
and are called MDI-X (the X for crossover), as shown in Figure B-1
. All
EtherSpeedII switch module 10BASE-T ports are MDI-X standard.
Two simple rules govern all usual RJ-45 connections from each switch module
10BASE-T/100BASE-TX port to the 10BASE-T/100BASE-TX port on another
device:
• A straight-through cable (MDI to MDI-X) is customarily used for connection
to a station NIC, to a transceiver, or to a cable plant patch panel.
• A crossover cable or adapter (MDI-X to MDI-X; as shown in Figure B-2
) is
customarily used for connection to another switch or hub port.
Figure B-2. MDI-X to MDI-X External Crossover (RJ-45 Example)
Making the crossover externally with a crossover cable or adapter that exchanges
the TD and RD wire pairs (as shown in Figure B-2
) is possible but has the
following disadvantages:
• A crossover cable is not standard. You may need to special order one or make
your own.
• Crossover cables and standard cables that resemble one another in a network
can cause confusion.
Caution: Make any crossover cable you use look obviously different from
normal straight-through cables and put a tag on each end; for example, use a
distinctive color like yellow and put a tag labeled “X-over” on each end.
Transmit
Receive
Transmit
Receive
MDI-X port
MDI-X port
9730EA
1
2
3
6
1
2
3
6
Twisted
pairs