display panel

Pin mappings:

Photo
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Description

The pins of an 8P8C jack are numbered 1-8 from left to right, as shown to the left (bottom center).

The rows of header on the shield are numbered from bottom to top, as shown upper left and upper right.

If you follow the traces, you'll see that the row numbers don't correspond to the 8P8C pin numbers.

You may wonder why I broke the correlation between row numbers and pin numbers. The picture to the left is the reason.

We humans have a strong tendency to find patterns, and once we find them it's hard to make us let go. It's very difficult to look at the numbering scheme on the left and not think that the sequence of rows runs vertically. We prefer the 5 that's right next to the 4, even though we know that the correct one is across in the next row.

After about the fifth time I tripped myself that way, I decided to change the numbering.

As long as you're working with the breadboard headers, the 8P8C pin numbering is just a low-level hardware standard you can safely ignore. Row 5 of any port on the shield corresponds to row 5 of any header, and vice versa.

Hardware hackers have a lousy history of coloring inside the lines though, so here are the mappings you'll need if you want to play with the cables themselves:

8P8C -vs- Row numbers:

8P8C 1 2 3 4 5 6 7 8
Row 1 2 3 4 8 7 6 5

Row 1 2 3 4 5 6 7 8
8P8C 1 2 3 4 8 7 6 5

CAT5 cable numbers:

If you want to work with the wires themselves, things get more complicated. There are two separate-but-equal standards for wiring Ethernet cables, and neither one corresponds to the cable pair numbers.

Let's start off with the cables themselves:

Pair Wire Color
1 1 White/Blue
1 2 Blue
2 1 White/Orange
2 2 Orange
3 1 White/Green
3 2 Green
4 1 White/Brown
4 2 Brown

The white/color wire of any pair is called the 'tip' line. The solid-color wire of any pair is called the 'ring' line.

The terms date back to the plugs used in telephone switchboards.. three-connector audio plugs have (obviously) three connectors called the 'tip', 'ring', and 'sleeve', which pretty much describe what they are. 'Tip' is the pointed part that you push into the jack first. 'Sleeve' is the barrel that connects to the part you hold. 'Ring' is the short connector between the tip and the sleeve, and is in fact a small ring of metal.

Tip was the ground wire, ring carried the signal.

T568-A:

The first standard for wiring Ethernet cables is T568-A:

8P8C # Cat5 # Shield # Color
1 3/1 1 White/Green
2 3/2 2 Green
3 2/1 3 White/Orange
4 1/2 4 Blue
5 1/1 8 White/Blue
6 2/2 7 Orange
7 4/1 6 White/Brown
8 4/2 5 Brown

Rearranged for shield row numbers:

8P8C # Cat5 # Shield # Color
1 3/1 1 White/Green
2 3/2 2 Green
3 2/1 3 White/Orange
4 1/2 4 Blue
8 4/2 5 Brown
7 4/1 6 White/Brown
6 2/2 7 Orange
5 1/1 8 White/Blue

and for cable numbers:

8P8C # Cat5 # Shield # Color
5 1/1 8 White/Blue
4 1/2 4 Blue
3 2/1 3 White/Orange
6 2/2 7 Orange
1 3/1 1 White/Green
2 3/2 2 Green
7 4/1 6 White/Brown
8 4/2 5 Brown

T568-B:

The second standard for wiring Ethernet cables is T568-B. It's electrically equivalent to T568-A, it just swaps pairs 2 & 3:

8P8C # Cat5 # Shield # Color
1 2/1 1 White/Orange
2 2/2 2 Orange
3 3/1 3 White/Green
4 1/2 4 Blue
5 1/1 8 White/Blue
6 3/2 7 Green
7 4/1 6 White/Brown
8 4/2 5 Brown

Rearranged for shield row numbers:

8P8C # Cat5 # Shield # Color
1 2/1 1 White/Orange
2 2/2 2 Orange
3 3/1 3 White/Green
4 1/2 4 Blue
8 4/2 5 Brown
7 4/1 6 White/Brown
6 3/2 7 Green
5 1/1 8 White/Blue

And for cable numbers:

8P8C # Cat5 # Shield # Color
5 1/1 8 White/Blue
4 1/2 4 Blue
1 2/1 1 White/Orange
2 2/2 2 Orange
3 3/1 3 White/Green
6 3/2 7 Green
7 4/1 6 White/Brown
8 4/2 5 Brown

T58-B cables tend to be more common than T58-A cables, but it's still easiest to check your connections with a continuity tester.

Crossover cables:

A crossover cable is wired for T568-A on one end, T568-B on the other end.

If you have a T568-A cable, Ethernet transmits data on pair 2 (8P8C pins 1 & 2), and receives data on pair 3 (8P8C pins 3 & 6). If it's a T568-B cable, TX goes over pair 3 and RX comes over pair 2.

Network switches and hubs swap the RX and TX signals internally, so what one device sends out on pair 2 (T568-A standard) automagically comes into all other devices on pair 3.

NOTE: if you connect a patch shield to cables that go through hubs or switches, your signals will get swapped.

Newer Ethernet cards can swap pairs 2 & 3 themselves if necessary, but early cards weren't that smart. You *needed* a hub or a switch between the two machines to swap the TX and RX signals. Plugging a straight-through cable between two machines wouldn't work because one of the machines was basically holding the phone upside down.

A crossover cable swaps the TX and RX signals within the cable itself, thus overcoming the weaknesses of early Ethernet cards, allowing machines with said cards to talk to each other.

Other standards:

They're out there.

RJ11, RJ14, and RJ25 are 2, 4, and 6-pin phone standards that are sometimes wired in 8P8C connectors. Only RJ11 is simple, and that's just because there are only so many ways you can screw around with two wires. It puts tip on 8P8C pin 5 and ring on pin 4.

RJ14 adds another pair outside the RJ11 pair.. tip is on 8P8C pin 3, ring is on pin 6. T568-A was defined to make 8P8C pins 3-6 compatible with RJ14 wiring, but T568-B is incompatible with RJ14.

RJ25 adds yet another pair outside RJ14, tip on 8P8C pin 2, ring on pin 7.

RJ45 is basically RJ11 with a resistor connected between 8P8C pins 7 & 8.

RJ48 has ring lines on 8P8C pins 1, 3, 4 & 8, and tip lines on 2, 5, 6 & 7. RJ48C is the crossover cable for RJ48, and swaps pins 1, 2, 4 & 5.

RJ61 has tip lines on 8P8C pins 1,2,3 & 5, ring lines on 8P8C pins 4, 6, 7 & 8.

The upshot of all this is that there's no simple way to map the wires in a randomly-selected cable to the rows on the patch shield. If you want to work at the physical connection layer -- and there's no reason you shouldn't -- you'll need to check the connections and draw yourself a map.