What are the functions of the 8 pins of an RJ45 connector?

I. Pin Numbering and Standard Wiring

II. Pin Functions at Different Network Speeds

1. 10BASE-T/100BASE-TX (100Mbps Network)

• Data Transmission Pins: Only 4 wires are used (1/2/3/6)
  • 1/2: Form the transmit differential pair (TX+/-), used for device data transmission;
  • 3/6: Form the receive differential pair (RX+/-), used for device data reception;
• Spare Pins: 4/5/7/8 are usually grounded or unused. In some cases, they are used to connect telephone lines (e.g., VoIP devices).

2. 1000BASE-T (Gigabit Network)

• Full-duplex transmission: Uses all 8 wires (4 pairs of differential signals)
  • 1/2: Transmit differential pair 1 (TX1+/-);
  • 3/6: Receive differential pair 1 (RX1+/-);
  • 4/5: Transmit/Receive differential pair 2 (TX2/RX2+/-);
  • 7/8: Transmit/Receive differential pair 3 (TX3/RX3+/-);
• Note: In Gigabit networks, each pair of pins simultaneously handles transmission and reception, achieving bidirectional transmission through full-duplex technology.

3. PoE (Power over Ethernet) Scenarios

• 802.3af/at standard (PoE+):
  • Power Pins: Power is transmitted through 4/5 (positive) and 7/8 (negative), or 1/2/3/6 simultaneously transmits data and power (pin allocation varies depending on the power supply mode);
  • For example: Mode A uses 1/2 (+) and 3/6 (-), Mode B uses 4/5 (+) and 7/8 (-). Wiring needs to be based on the PoE type supported by the device.

III. Comparison of Pin Functions between T568A and T568B

IV. Engineering Application Points for Pin Functions

• Differential pairs (such as 1/2, 3/6) should maintain a twisted state to reduce electromagnetic interference (EMI). Unused pins are recommended to be grounded;
• In Gigabit networks, the twist pitch of the four twisted pairs needs to match the cable type (e.g., CAT6a requires a twist pitch ≤0.5mm).

• Confirm the PoE mode (A/B) supported by the device to avoid reverse connection causing device damage;
• The 802.3at standard has a maximum power supply of 30W. When power is supplied through pins 4/5 and 7/8, ensure that the copper core cross-sectional area of the wire is ≥0.2mm².

• 100Mbps network failures are often caused by poor contact of pins 1/2/3/6. A network cable tester can be used to check the continuity of the corresponding pins;
• Gigabit networks require all 8 wires to be connected. If speed negotiation fails, it may be due to an open circuit in one of the 4/5/7/8 wires.

V. Matching of Pin Functions and Cable Types

Summary