Ethernet RJ45 — T568A and T568B
Definition
Ethernet BASE-T links use an 8P8C modular connector, commonly called RJ45, and four twisted pairs. T568A and T568B define the conductor colour order at the connector; they do not define the Ethernet speed, cable category, or PoE polarity.
Orientation and numbering
The illustrations show the contacts visible, the cable pointing down, and pins 1 through 8 from left to right. Turning the plug over reverses the apparent order; a receptacle viewed from the front may also appear mirrored. Always follow the manufacturer’s view and numbering when checking a PCB footprint.
Pinout, colours, and signals
| Pin | T568A | T568B | Twisted pair | 10/100BASE-T signal on an MDI port | 1000BASE-T and faster signal |
|---|---|---|---|---|---|
| 1 | White/green | White/orange | 1–2 | TX+ | BI_DA+ |
| 2 | Green | Orange | 1–2 | TX− | BI_DA− |
| 3 | White/orange | White/green | 3–6 | RX+ | BI_DB+ |
| 4 | Blue | Blue | 4–5 | Not used | BI_DC+ |
| 5 | White/blue | White/blue | 4–5 | Not used | BI_DC− |
| 6 | Orange | Green | 3–6 | RX− | BI_DB− |
| 7 | White/brown | White/brown | 7–8 | Not used | BI_DD+ |
| 8 | Brown | Brown | 7–8 | Not used | BI_DD− |
T568A and T568B swap only the orange and green pairs. Conductors 3 and 6 are one twisted pair, even though contacts 4 and 5 lie between them. Keeping the correct pairs together and limiting untwist at the termination are essential to preserve impedance and noise rejection.
Pins used by each Ethernet type
| Technology | Nominal rate | Data pairs | Connector use |
|---|---|---|---|
| 10BASE-T | 10 Mbit/s | 2 | 1–2 and 3–6, separate transmit and receive on a legacy MDI port |
| 100BASE-TX | 100 Mbit/s | 2 | 1–2 and 3–6, separate transmit and receive on a legacy MDI port |
| 1000BASE-T | 1 Gbit/s | 4 | 1–2, 3–6, 4–5, and 7–8, bidirectional signalling on every pair |
| 2.5GBASE-T | 2.5 Gbit/s | 4 | All four pairs, bidirectional signalling |
| 5GBASE-T | 5 Gbit/s | 4 | All four pairs, bidirectional signalling |
| 10GBASE-T | 10 Gbit/s | 4 | All four pairs, bidirectional signalling |
| 25GBASE-T | 25 Gbit/s | 4 | All four pairs, Cat 8 / Class I or II application up to 30 m |
| 40GBASE-T | 40 Gbit/s | 4 | All four pairs, Cat 8 / Class I or II application up to 30 m |
The T568A/B pinout does not change as the rate increases. The PHY, channel quality, and cabling category determine which mode can be negotiated. With 1000BASE-T and faster links, no pair is spare for data, even when PoE power simultaneously travels in common mode.
25GBASE-T and 40GBASE-T target a 30 m four-pair Class I or Class II channel. The 8P8C interface illustrated here corresponds to the Class I / Category 8.1 family; a Class II / Category 8.2 channel may use another connector interface whose mechanical pinout must be checked separately.
The table gives TX/RX names from the viewpoint of a legacy 10/100BASE-T MDI port. An MDI-X port reverses those roles. They must therefore not be assigned universally to the orange or green colour pair.
Straight-through and crossover cables
Straight-through cable
A straight-through cable uses the same scheme at both ends: T568A–T568A or T568B–T568B. Every pin connects to the same pin number, from 1→1 through 8→8. This is the default patch-cable arrangement.
Legacy 10/100BASE-T crossover
A cable terminated as T568A at one end and T568B at the other swaps the two data pairs used by 10BASE-T and 100BASE-TX:
| End 1 pin | 10/100 MDI signal | End 2 pin | Result |
|---|---|---|---|
| 1 | TX+ | 3 | Crossed to RX+ |
| 2 | TX− | 6 | Crossed to RX− |
| 3 | RX+ | 1 | Crossed to TX+ |
| 4 | Not used | 4 | Straight through |
| 5 | Not used | 5 | Straight through |
| 6 | RX− | 2 | Crossed to TX− |
| 7 | Not used | 7 | Straight through |
| 8 | Not used | 8 | Straight through |
This crossover was used, among other cases, to connect two like devices that did not support Auto MDI-X.
True four-pair crossover
To cross all four pairs explicitly for legacy 1000BASE-T equipment, the two additional pairs must also be exchanged:
| End 1 | End 2 |
|---|---|
| 1 | 3 |
| 2 | 6 |
| 3 | 1 |
| 6 | 2 |
| 4 | 7 |
| 5 | 8 |
| 7 | 4 |
| 8 | 5 |
A simple T568A–T568B cable is therefore only a two-pair crossover, not a complete four-pair crossover. Most modern ports detect and correct pair swapping with Auto MDI-X; check the documentation for old or specialised equipment before making a crossover cable.
Practical checks
- Use one termination scheme consistently throughout structured cabling unless a documented crossover is required.
- Test continuity, all eight conductor positions, and correct pair grouping; a simple pin-to-pin tester does not certify high-frequency performance.
- Select cable category, shielding where needed, and length for the intended rate, EMC environment, and PoE load.
- Do not confuse T568A/T568B with PoE Alternatives A/B: the former specify colours, while the latter identify powered pairsets.
Related documentation
- Straight-through and crossover Ethernet cables
- Ethernet cable categories from Cat 5 to Cat 8
- Power over Ethernet: PoE, PoE+, and 4PPoE
- Ethernet BASE-T rates from 10 Mbit/s to 40 Gbit/s
Official and technical sources
- ANSI/TIA-568.2-E — balanced twisted-pair cabling standard announcement
- Leviton — T568A and T568B markings for Cat 5e/Cat 6 connectors
- IEEE 802.3-2022 — Standard for Ethernet
- IEEE 802.3bz — four-pair 2.5GBASE-T and 5GBASE-T objectives
- IEEE 802.3bq — 25GBASE-T and 40GBASE-T up to 30 m
- Cisco — straight-through and crossover 10/100BASE-T pinouts
- Cisco — four-pair 1000BASE-T crossover pinout, Table A-26
- Cisco — Auto MDI-X operation