Frequently Asked Questions
ETHERNET QUESTIONS
13) WHAT IS THE MDI/MDI-X?
Answer: MDI are the Ethernet Medium Dependent Interface (MDI), and the Medium Dependent Interface Crossover (MDI-X) specifications. The specifications define the electrical and connection requirements for devices connected with twisted-pair wire Ethernet cables, such as CAT5, CAT5e, or CAT6 cable and 8P8C (RJ45) connectors. The terms MDI and MDI-X define the pinouts on the physical connectors mounted in the devices (computers, switches, routers, etc.). MDI has Transmit+ and Transmit- on pins 1 and 2, with Receive+ and Receive- on pins 3 and 6. MDI-X has Receive+ and Receive- on pins 1 and 2 with Transmit+ and Transmit- on pins 3 and 6. Note that a straight-through wired cable mates the Transmit lines of the MDI device to the Receive lines on the MDI-X device, and the Receive lines of the MDI device to the Transmit lines on the MDI-X device. Typically, a computer is wired in the MDI configuration with a router wired in the MDI-X configuration, so a straight-through wired cable mates up the signal lines of a computer to a router properly. Newer devices may use Auto MDI-X ports which automatically reconfigure the signals to properly match the other end of the link. The pinouts for the MDI and MDI-X connectors are the following:
| PIN | MDI SIGNAL | MDI-X SIGNAL |
|---|---|---|
| 1 | TX+ | RX+ |
| 2 | TX- | RX- |
| 3 | RX+ | TX+ |
| 4 | Not Connected | Not Connected |
| 5 | Not Connected | Not Connected |
| 6 | RX- | TX- |
| 7 | Not Connected | Not Connected |
| 8 | Not Connected | Not Connected |
