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Field guide · Cabling & termination

RJ45 wiring

The two eight-position colour codes in full, with an interactive pinout you can flip between. Shows which pins actually move between A and B, which pairs carry data on 100 megabit versus gigabit, and how a crossover cable is built — plus why you almost certainly do not need one.

Plate 01Diagram

Both colour codes, pin by pin, side by side.

Colour code

The usual choice in US commercial work.

Cable type

Same code at both ends. What you want, essentially always.

Both ends — T568B

Held with the clip away from you and the contacts facing you, position 1 is on the left.

What each pair carries
10BASE-T / 100BASE-TXPositions 1, 2, 3 and 6 only

The blue and brown pairs sit idle. A run with two damaged pairs still links at 100 megabit, which is why a bad cable can hide for months.

1000BASE-TAll eight positions

Every pair carries data in both directions at once. This is the speed at which a split pair or a crushed conductor finally shows itself.

PoEEither pair set, or all four

802.3af and at power over two pairs; 802.3bt uses all four to reach 90 W.

Select a position for its pair number and what it carries. Positions marked in hi-vis are the four that change between T568A and T568B — the orange and green pairs trading places.

T568B pin assignments
PinConductorPair10/100Gigabit
1white/orange2TX+BI_DA+
2orange2TX−BI_DA−
3white/green3RX+BI_DB+
4blue1unusedBI_DC+
5white/blue1unusedBI_DC−
6green3RX−BI_DB−
7white/brown4unusedBI_DD+
8brown4unusedBI_DD−
Fig. 01The eight-position pinout in both TIA-568 colour codes. Positions marked in hi-vis are the four that change between them — the orange and green pairs trading places, with blue and brown staying put.
Plate 02Drill

Now do it
without looking

The sequence is worth knowing cold — on a ladder with a torch in your teeth, looking it up is the slow part. Place all eight and check.

Wire it to

The usual choice in US commercial work.

Wire it yourself

Place all eight conductors in T568B order, position 1 on the left. Hold the plug with the clip away from you and the contacts facing you.

Select a conductor, then the position it belongs in. Select a filled position to lift it back out. Nothing is timed until you start.

Plate 03Procedure

Doing it

Terminating a plug, start to finish. The same sequence applies to a jack, except that you punch rather than crimp — see the keystone guide for that.

Tools
  • Crimp tool for eight-position plugs
  • Cable jacket stripper
  • Cable tester
Materials
  • Cat5e or Cat6 cable
  • Eight-position modular plugs
  1. 01

    Pick one standard and commit

    T568B is the usual choice in US commercial work; T568A appears in federal and residential specifications. Neither performs better. Decide once and use it on every jack, panel, and plug in the building.

  2. 02

    Strip about an inch of jacket

    Score the jacket lightly and flex it off rather than cutting deep. Nicking a conductor's insulation causes a fault that passes a continuity test and fails certification.

    CautionIf you nick a conductor, cut back and start again. It will not get better inside the wall.

  3. 03

    Untwist only as much as you need

    Separate the four pairs, straighten them, and fan them into the colour order for your standard. Keep the untwisted length under half an inch — this is the single largest thing you control in the crosstalk measurement.

  4. 04

    Trim square and seat fully

    Cut all eight conductors to the same length in one pass. Slide them into the plug with the clip facing away from you and check through the plug body that every conductor is touching the front wall before crimping.

    CautionA conductor short of the front wall makes intermittent contact — the worst fault to chase later.

  5. 05

    Crimp, then test

    Press the crimp fully home so the strain relief bites on the jacket, not on the conductors. Then put a tester on it. An untested termination is an assumption.

Plate 04Questions
Questions people actually ask
Does it matter whether I use T568A or T568B?
Not for performance — the two codes are electrically identical, differing only in which pair sits on which pins. What matters is consistency. Mixing them within one building produces accidental crossover cables that behave unpredictably on older equipment.
What is the difference between T568A and T568B?
The orange and green pairs swap places. In T568B the orange pair sits on pins 1 and 2 and green on 3 and 6; in T568A green is on 1 and 2 and orange on 3 and 6. The blue and brown pairs do not move.
Do I still need a crossover cable?
Almost never. Auto-MDI/MDI-X is mandatory on 1000BASE-T and has been standard on switch ports for two decades, so the port sorts out transmit and receive itself. Only genuinely old equipment still needs one.
Why do only four of the eight wires carry data?
On 10BASE-T and 100BASE-TX only pins 1, 2, 3 and 6 are energised. 1000BASE-T uses all four pairs bidirectionally, which is why a damaged blue or brown pair can leave a link working at 100 megabit while refusing to negotiate gigabit.
Plate —Other guides

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