Low Frequency RF Absorbers 500 MHz–3 GHz | Wavexorb
Wednesday, August 12, 2026
Crossover Ethernet Cables:
Do You Still Need Crossover Ethernet Cables in 2025? - Cables and Kits - Learning Center
Ethernet RJ45 jacks and plugs follow two primary wiring standards defined by ANSI/TIA-568: T568A and T568B.
Both standards use the exact same 8-conductor cable and deliver identical electrical performance.
RJ45 Pinout Comparison Table
When looking at the front (gold pins facing you) or looking down at the RJ45 plug with the locking clip facing away/down, the pins are numbered 1 through 8 from left to right.
| Pin Number | T568A Color Code | T568B Color Code | Pair Function (10/100/1000 Mbps) |
| 1 | White/Green | White/Orange | Transmit / Receive (Pair 3 / Pair 2) |
| 2 | Green | Orange | Transmit / Receive (Pair 3 / Pair 2) |
| 3 | White/Orange | White/Green | Transmit / Receive (Pair 2 / Pair 3) |
| 4 | Blue | Blue | Bi-directional / PoE (Pair 1) |
| 5 | White/Blue | White/Blue | Bi-directional / PoE (Pair 1) |
| 6 | Orange | Green | Transmit / Receive (Pair 2 / Pair 3) |
| 7 | White/Brown | White/Brown | Bi-directional / PoE (Pair 4) |
| 8 | Brown | Brown | Bi-directional / PoE (Pair 4) |
Key Rules for Implementation
- Consistency is Critical: Both ends of a standard patch cable must use the same wiring standard (either both T568A or both T568B) to create a Straight-Through cable.
Mixing them on opposite ends creates a Crossover cable. (Note: Modern network equipment features Auto-MDI/MDIX, making crossover cables largely obsolete for device-to-device uplinks.) - T568B is the most widely deployed standard globally for commercial, enterprise, and residential installations.
- T568A is sometimes mandated for U.S. residential or federal government projects due to legacy telephone (USOC) backward-compatibility requirements.
| Pin Number | T568A Color Code | T568B Color Code | Pair Function (10/100/1000 Mbps) |
| 1 | White/Green | White/Orange | Transmit / Receive (Pair 3 / Pair 2) |
| 2 | Green | Orange | Transmit / Receive (Pair 3 / Pair 2) |
| 3 | White/Orange | White/Green | Transmit / Receive (Pair 2 / Pair 3) |
| 4 | Blue | Blue | Bi-directional / PoE (Pair 1) |
| 5 | White/Blue | White/Blue | Bi-directional / PoE (Pair 1) |
| 6 | Orange | Green | Transmit / Receive (Pair 2 / Pair 3) |
| 7 | White/Brown | White/Brown | Bi-directional / PoE (Pair 4) |
| 8 | Brown | Brown | Bi-directional / PoE (Pair 4) |
For a standard non-integrated RJ45 jack connecting to external magnetics, the standard Ethernet differential pairs (A, B, C, and D) map to the physical connector pin numbers as follows:
| Pin Number | Standard Signal Assignment | Pair Function (10/100/1000 Mbps) |
| 1 | TRRXA_P (Pair A - Positive) | Transmit / Receive |
| 2 | TRRXA_N (Pair A - Negative) | Transmit / Receive |
| 3 | TRRXB_P (Pair B - Positive) | Transmit / Receive |
| 4 | TRRXC_P (Pair C - Positive) | Bi-directional / PoE |
| 5 | TRRXC_N (Pair C - Negative) | Bi-directional / PoE |
| 6 | TRRXB_N (Pair B - Negative) | Transmit / Receive |
| 7 | TRRXD_P (Pair D - Positive) | Bi-directional / PoE |
| 8 | TRRXD_N (Pair D - Negative) | Bi-directional / PoE |
When routing the Medium Dependent Interface (MDI) traces from the Ethernet PHY chip to the external discrete magnetics (and onward to the RJ45 jack), strict length matching and impedance controls are required to prevent signal reflections, common-mode noise, and EMI failures.
1. Trace Length Limits
- Maximum Distance: Keep the total trace length from the PHY pins through the magnetics to the RJ45 connector as short as possible—ideally under 1 to 2 inches (max 2 to 4 inches depending on the specific PHY manufacturer datasheet). Longer traces increase signal attenuation and radiation.
2. Length Matching Tolerances (Skew Control)
- Intra-Pair Matching (Positive vs. Negative): The individual positive (
_P) and negative (_N) traces within the same differential pair must be tightly matched to avoid signal skew that creates common-mode noise.- 1000BASE-T (Gigabit): Match within 20 mils (0.5 mm).
- 100BASE-TX / 10BASE-T: Match within 50 mils (1.25 mm).
- Inter-Pair Matching (Pair-to-Pair): While matching lengths between different pairs (e.g., Pair A vs. Pair B) is less critical for gigabit auto-negotiation timing than intra-pair matching, manufacturers typically recommend keeping all four pairs roughly balanced (often within 1,000 mils of each other) to maintain uniform propagation delays.
3. Impedance Requirements
- Differential Impedance: Each of the four pairs must target 100 Ohms (+/- 10% to 15%) differential impedance.
- Single-Ended Impedance: Approximately 50 Ohms per individual trace.
4. Critical Layout Best Practices
- Symmetry: Route the differential pairs symmetrically, maintaining a constant gap between the
_Pand_Ntracks. - Reference Planes: Ensure an uninterrupted, continuous ground plane reference directly underneath the entire MDI trace run. Never route high-speed Ethernet traces across split planes or plane voids (except for the required chassis/isolated ground split under the magnetics module itself).
- Vias: Minimize the use of vias. Limit them to a absolute maximum of 2 vias per trace across the entire MDI path to prevent impedance discontinuities.
Ethernet Magnetics Pinout Breakdown
RJ45 connector with integrated magnetics)
For Gigabit (1000BASE-T) Ethernet, the four differential pairs are labeled using letters (A, B, C, D) alongside positive (
_P) and negative (_N) polarities.| Pad / Pin | Signal Name | Differential Pair & Function |
| 1 | ETH1_TRRXA_P | Pair A (Positive) |
| 2 | ETH1_TRRXA_N | Pair A (Negative) |
| 3 | ETH1_TRRXB_P | Pair B (Positive) |
| 4 | ETH1_TRRXC_P | Pair C (Positive) |
| 5 | ETH1_TRRXC_N | Pair C (Negative) |
| 6 | ETH1_TRRXB_N | Pair B (Negative) |
| 7 | ETH1_TRRXD_P | Pair D (Positive) |
| 8 | ETH1_TRRXD_N | Pair D (Negative) |
| 9 / 11 | LED_1_1000 / LED_1_ACT | Status LEDs (Speed / Activity) |
Important Layout Note for Integrated Magnetics Jacks
Notice how the pin pairs on the PHY/transformer side do not always follow a sequential 1-2, 3-4 order spatially (for instance, Pair C and Pair B pins are interleaved on pins 3 through 6). This is completely normal for integrated RJ45 magnetic modules; the internal coil layout routes the pins to optimize crossover minimization and match the physical pinout of the Ethernet transceiver (PHY) chip. Always cross-reference the specific footprint datasheet for your exact jack model to ensure proper trace routing.
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