thumb|upright=1.25|Single-core 25-pair/50 conductor cable
The 25-pair color code, originally known as even-count color code, is a color code used to identify individual conductors in twisted-pair wiring for telecommunications.
Color coding
With the development of new generations of telecommunication cables with polyethylene-insulated conductors (PIC) by Bell Laboratories for the Bell System in the 1950s, new methods were developed to mark each individual conductor in cables. Each wire is identified by the combination of two colors, one of which is the major color, and the second the minor color. Major and minor colors are chosen from two different groups of five, resulting in 25 color combinations. The color combinations are applied to the insulation that covers each conductor. Typically, one color is a prominent background color of the insulation, and the other is a tracer, consisting of stripes, rings, or dots, applied over the background. The background color always matches the tracer color of its paired conductor, and vice versa.
The major, or primary group of colors consists of the sequence of white, red, black, yellow, and violet (mnemonics Why Run Backwards, You'll Vomit). The minor, or secondary color is chosen from the sequence blue, orange, green, brown, and slate (mnemonics Bell Operators Give Better Service).
thumb|upright=1.25|25-pair color coding using twisted pairs with solid color coding only, without tracers. This method was rarely used.
{| class="wikitable"
|+25-pair color code
! Pair<br/>no. !!colspan="2"| Major color !!colspan="2"| Minor color
|-
|align="center"| 1 ||rowspan="5" align="right" vertical-align="middle"| White ||rowspan="5" bgcolor="white"| ||bgcolor="blue"| || Blue
|-
|align="center"| 2 ||bgcolor="orange"| || Orange
|-
|align="center"| 3 ||bgcolor="#00FF00"| || Green
|-
|align="center"| 4 ||bgcolor="brown"| || Brown
|-
|align="center"| 5 ||bgcolor="slategray"| || Slate
|-
|align="center"| 6 ||rowspan="5" align="right" vertical-align="middle"| Red ||rowspan="5" bgcolor="red"| ||bgcolor="blue"| || Blue
|-
|align="center"| 7 ||bgcolor="orange"| || Orange
|-
|align="center"| 8 ||bgcolor="#00FF00"| || Green
|-
|align="center"| 9 ||bgcolor="brown"| || Brown
|-
|align="center"| 10 ||bgcolor="slategray"| || Slate
|-
|align="center"| 11 ||rowspan="5" align="right" vertical-align="middle"| Black ||rowspan="5" bgcolor="black"| ||bgcolor="blue"| || Blue
|-
|align="center"| 12 ||bgcolor="orange"| || Orange
|-
|align="center"| 13 ||bgcolor="#00FF00"| || Green
|-
|align="center"| 14 ||bgcolor="brown"| || Brown
|-
|align="center"| 15 ||bgcolor="slategray"| || Slate
|-
|align="center"| 16 ||rowspan="5" align="right" vertical-align="middle"| Yellow ||rowspan="5" bgcolor="yellow"| ||bgcolor="blue"| || Blue
|-
|align="center"| 17 ||bgcolor="orange"| || Orange
|-
|align="center"| 18 ||bgcolor="#00FF00"| || Green
|-
|align="center"| 19 ||bgcolor="brown"| || Brown
|-
|align="center"| 20 ||bgcolor="slategray"| || Slate
|-
|align="center"| 21 ||rowspan="5" align="right" vertical-align="middle"| Violet ||rowspan="5" bgcolor="violet"| ||bgcolor="blue"| || Blue
|-
|align="center"| 22 ||bgcolor="orange"| || Orange
|-
|align="center"| 23 ||bgcolor="#00FF00"| || Green
|-
|align="center"| 24 ||bgcolor="brown"| || Brown
|-
|align="center"| 25 ||bgcolor="slategray"| || Slate
|}
The wire pairs are referenced directly by their color combination, or by the pair number. For example, pair 9 is also called the red-brown pair. In technical tabulations, the colors are often suitably abbreviated.
Violet is the standard name in the telecommunications and electronics industry, but it is sometimes referred to as purple. Similarly, slate is a particular shade of gray. The names of most of the colors were taken from the conventional colors of the rainbow or optical spectrum, and in the electronic color code, which uses the same ten colors, albeit in a different order.
When used for plain old telephone service (POTS), the first wire is known as the tip or A-leg (U.K.) conductor, and is usually connected to the positive side of a direct current (DC) circuit, while the second wire is known as the ring lead or B-leg (U.K.), and is connected to the negative side of the circuit. Neither of these two sides of the line has a connection to the local ground. This creates a balanced audio circuit with common-mode rejection, also known as a differential pair. The tip and ring convention is based on the ″ (6.5 mm) TRS phone connectors, which were employed in telephone switchboards in the 19th and 20th centuries, where the tip contact of the connector is separated from the ring contact by a spacer of insulation. The connection furthest from the cable is known as the tip, the middle connection is the ring, and the (largest) connection closest to the wire is the sleeve.
25-pair telco cable pinout
A common application of the 25-pair color code is the cabling for the Registered Jack interface RJ21, which uses a female 50-pin miniature ribbon connector, as shown in the following table. The geometry of the pins of the receptacle (right hand image) corresponds to the pin numbers of the table. The left column of pins are the ring (R) conductors, while all tip (T) conductors are on the right.
{| class="wikitable"
! rowspan=2| Color<br /><small>(minor/major)</small> || (R) || || (T) || rowspan=2 | Color<br /><small>(major/minor)</small> || rowspan=2 | The corresponding<br/>pin order in the<br/>female RJ21 connector
|-
! colspan=3|Pin No.
|-
| 37px blue/white
| 1
|
| 26
| 37px white/blue
| rowspan=31 style="vertical-align: top;" | 160px
|-
| 37px orange/white
| 2
|
| 27
| 37px white/orange
|-
| 37px green/white
| 3
|
| 28
| 37px white/green
|-
| 37px brown/white
| 4
|
| 29
| 37px white/brown
|-
| 37px slate/white
| 5
|
| 30
| 37px white/slate
|-
| colspan=5 |
|-
| 37px blue/red
| 6
|
| 31
| 37px red/blue
|-
| 37px orange/red
| 7
|
| 32
| 37px red/orange
|-
| 37px green/red
| 8
|
| 33
| 37px red/green
|-
| 37px brown/red
| 9
|
| 34
| 37px red/brown
|-
| 37px slate/red
| 10
|
| 35
| 37px red/slate
|-
| colspan=5 |
|-
| 37px blue/black
| 11
|
| 36
| 37px black/blue
|-
| 37px orange/black
| 12
|
| 37
| 37px black/orange
|-
| 37px green/black
| 13
|
| 38
| 37px black/green
|-
| 37px brown/black
| 14
|
| 39
| 37px black/brown
|-
| 37px slate/black
| 15
|
| 40
| 37px black/slate
|-
| colspan=5 |
|-
| 37px blue/yellow
| 16
|
| 41
| 37px yellow/blue
|-
| 37px orange/yellow
| 17
|
| 42
| 37px yellow/orange
|-
| 37px green/yellow
| 18
|
| 43
| 37px yellow/green
|-
| 37px brown/yellow
| 19
|
| 44
| 37px yellow/brown
|-
| 37px slate/yellow
| 20
|
| 45
| 37px yellow/slate
|-
| colspan=5 |
|-
| 37px blue/violet
| 21
|
| 46
| 37px violet/blue
|-
| 37px orange/violet
| 22
|
| 47
| 37px violet/orange
|-
| 37px green/violet
| 23
|
| 48
| 37px violet/green
|-
| 37px brown/violet
| 24
|
| 49
| 37px violet/brown
|-
| 37px slate/violet
| 25
|
| 50
| 37px violet/slate
|}
Larger cables
For cables with more than 25 pairs, each group of 25 is called a binder group. The binder groups are marked with mylar ribbons using the same color coding system, starting with a white/blue ribbon, then a white/orange ribbon, and so on. The 24th binder group has a violet/brown ribbon, completing a super binder of 600 pairs.
See also
- 20-pair colour code (Australia)