The RJ45 connector (more precisely 8P8C) is on every Ethernet cable. Its eight wires in four twisted pairs are terminated according to either the T568A or the T568B standard. Both work the same – they only differ in that the orange and green pairs are swapped.
The most important rule: both ends of the cable must use the same wiring. A B–B or A–A cable is straight-through; combining A and B gives a crossover cable.
Key specs
- 8 contacts (8P8C), 1.02 mm pitch, 11.7 mm connector width
- 4 twisted pairs – blue, orange, green and brown; each pair has a solid-color wire and a white wire with a colored stripe
- 10 and 100 Mb/s use only pins 1, 2, 3 and 6; gigabit and faster Ethernet use all four pairs
- PoE: 802.3af up to 15.4 W, 802.3at (PoE+) up to 30 W, 802.3bt up to 60 and 90 W
- Cat5e cable handles 1 Gb/s, Cat6A 10 Gb/s over 100 m
RJ45 Wiring: T568A and T568B
The plug is drawn from the contact side with the latch underneath – hold it with the contacts facing up and the tip pointing away from you, and pin 1 is on the left. Pin 1 is also on the left on the jack when you look into the opening with the latch at the bottom. Use the filter to highlight pairs, and hover to highlight a single pin.
| Pin | T568A | T568B | 10/100 Mb/s | 1 Gb/s | PoE (mód A, MDI-X) | PoE (mód B) |
|---|---|---|---|---|---|---|
| 1 | white/green | white/orange | TX+ | BI_DA+ | − | – |
| 2 | green | orange | TX− | BI_DA− | − | – |
| 3 | white/orange | white/green | RX+ | BI_DB+ | + | – |
| 4 | blue | blue | – | BI_DC+ | – | + |
| 5 | white/blue | white/blue | – | BI_DC− | – | + |
| 6 | orange | green | RX− | BI_DB− | + | – |
| 7 | white/brown | white/brown | – | BI_DD+ | – | − |
| 8 | brown | brown | – | BI_DD− | – | − |
Straight-Through and Crossover Cables
A straight-through (patch) cable has the same wiring on both ends and connects a computer, board or router to a switch. A crossover cable swaps the transmit and receive pairs – it used to be the way to connect two computers directly.
| Cable | End 1 | End 2 | Use |
|---|---|---|---|
| straight-through | T568B | T568B | computer, board or router – switch |
| straight-through | T568A | T568A | the same, just in different colors |
| crossover (10/100 Mb/s) | T568A | T568B | two devices directly, when neither supports Auto-MDI-X |
| gigabit crossover | T568A | white-orange, orange, white-green, white-brown, brown, green, blue, white-blue | also swaps the blue and brown pairs – practically pointless today |
Today’s network cards and switches have Auto-MDI-X and swap transmit and receive on their own, so you’ll almost never need a crossover cable. Just use the same wiring on both ends. Which one you choose is up to you – but stick to one within a single installation.
Which Pairs 100 Mb/s and Gigabit Use
10BASE-T and 100BASE-TX carry data on only two pairs: from the computer’s point of view, the pair on pins 1 and 2 transmits and the pair on pins 3 and 6 receives. Pins 3 and 6 form a pair even though they aren’t next to each other. The blue and brown pairs stay free, which is why they can carry power (passive PoE).
Gigabit (1000BASE-T) and faster Ethernet use all four pairs, in both directions at once. That’s why a cable with a broken wire often links at only 100 Mb/s.
Cable Categories Cat5e, Cat6, Cat6A, Cat7 and Cat8
The category specifies up to what frequency the cable has guaranteed parameters (attenuation and crosstalk) – and therefore what Ethernet speed it can carry. The 100 m length applies to the whole link: at most 90 m of fixed installation plus 10 m of patch cables.
| Category | Frequency | Speed | Length | Note |
|---|---|---|---|---|
| Cat5e (Class D) | 100 MHz | 1 Gb/s, 2.5 Gb/s | 100 m | most common, enough for gigabit |
| Cat6 (Class E) | 250 MHz | 5 Gb/s, 10 Gb/s | 100 m, 10 Gb/s only up to 55 m | – |
| Cat6A (Class EA) | 500 MHz | 10 Gb/s | 100 m | for new installations with headroom for 10 Gb/s |
| Cat7 (Class F) | 600 MHz | 10 Gb/s | 100 m | ISO standard only, shielded; GG45 or TERA connectors |
| Cat7A (Class FA) | 1000 MHz | 10 Gb/s | 100 m | ISO standard only; network equipment with its connectors isn’t sold |
| Cat8.1 (Class I) | 2000 MHz | 25 and 40 Gb/s | 30 m | data centers, RJ45 connector |
| Cat8.2 (Class II) | 2000 MHz | 25 and 40 Gb/s | 30 m | data centers, GG45, ARJ45 or TERA connectors |
Cat7 and Cat7A exist only in the international ISO/IEC 11801 standard; the American TIA-568 never adopted them. A cable sold as “Cat7” with ordinary RJ45 plugs is therefore really a Cat6A-level link. The older Cat5 (100 MHz) was replaced by Cat5e, and Cat3 (16 MHz) is used only for telephones today.
PoE – Power over Ethernet
Power over Ethernet carries a 44–57 V DC voltage over the cable in addition to data. The source (PSE – a switch or injector) first negotiates with the device (PD – a camera, access point) how much power it needs, and only then turns the voltage on.
| Standard | Designation | Source power | To device | Pairs |
|---|---|---|---|---|
| 802.3af | PoE (Type 1) | 15.4 W | 12.95 W | 2 |
| 802.3at | PoE+ (Type 2) | 30 W | 25.5 W | 2 |
| 802.3bt | PoE++ (Type 3) | 60 W | 51 W | 4 |
| 802.3bt | PoE++ (Type 4) | 90 W | 71.3 W | 4 |
- Mode A – the voltage runs over data pairs 1-2 and 3-6. It doesn’t interfere with data; the transformers in the device separate it.
- Mode B – the voltage runs over pairs 4-5 (+) and 7-8 (−), which 10/100 Mb/s doesn’t use.
- The source chooses which mode is used. The device must support both and tolerate reversed polarity.
- Passive PoE (MikroTik, Ubiquiti…) doesn’t negotiate with the device – the voltage (usually 24 or 48 V) is always on the cable, mostly on pairs 4-5 (+) and 7-8 (−).
What to watch out for
- Hold the plug with the contacts facing up and the tip pointing away from you. A flipped plug is the most common mistake – the wiring then comes out mirrored.
- Both ends of a straight-through cable must have the same wiring (B–B or A–A).
- Untwist the wires only as far as necessary, at most about 13 mm. A longer untwisted section degrades transmission, especially at gigabit speeds.
- Plugs for solid wire (installation cable) and for stranded wire (flexible patch cable) differ – the wrong plug makes unreliable contact.
- Don’t connect passive PoE to a device that doesn’t support it – the constantly applied voltage can damage it.
- Check the finished cable with a tester – it reveals swapped and broken wires.
Ethernet and PoE on chiptron.eu/
On chiptron.eu/ you'll find these articles and projects with Ethernet:
- ESP32-POE – Olimex board with 100 Mb/s Ethernet and PoE power
More pinouts
See all pinouts on the Pinouts page.