DATE OF CREATION  : APRIL 25,1996
LAST DATE MODIFIED: AUGUST 19,2004
Before sending me nasty e-mail on what is wrong with this document, please
check to make sure that you have the latest copy.
The original copy is/was served from
http://mike.passwall.com/class/public/ethernet/10T.txt
If you do have the latest copy and still find something wrong with it,
drop me a line telling me what is wrong, and I will try to change it
as soon as possible.
dugan@passwall.com          (Mike)

My very own docs on 10T ethernet connections:

It is possible to connect two computers without a hub together.

First let me start by giving a few definitions.
There is a numbering system used to identify wires. There is also a color
coding system to identify wires. It is possible to use any number to color
assignment you want in 10T jacks, just as long as you remain consistent.
There are 8 different numbers (1, 2, 3, 4, 5, 6, 7, and 8)
There are 8 Different color codes on the wires:

There are two common competing assignments for colors and numbers for RJ45
connective cables: (T568A and T568B):
Pin #             T568A              T568B
1                 White-Green        White-Orange
2                 Green              Orange
3                 White-Orange       White-Green
4                 Blue               Blue
5                 White-Blue         White-Blue
6                 Orange             Green
7                 White-Brown        White-Brown
8                 Brown              Brown

Color numbers by pairs:
Pair#                                Color
1                                    Blue (BLU)
2                                    Orange (ORG)
3                                    Green (GRN)
4                                    Brown (BRN)

Now, to identify number and wire colors assignments on the jacks.
Different ASCII renderings of a 10T Jack with appropriate number assignments:
PINOUT FOR RJ45 CABLE CONNECTION:
---------------------------------

    FRONT-END         BACK-END                          TOP
                 (Cable inserted here)
    #--------#        #--------#         #------------------------------#
   1]--O     |       8]--O|^^^||        8|----------            |####|  |
   2]--O     |       7]--O|   ||        7|----------            | >< |  |
   3]--O    |---\    6]--O|]  | ----    6|----------            | >< |  |
   4]--O    ||  ||   5]--O|]  | ||| |   5|----------            | >< |  |
   5]--O    ||  ||   4]--O|]  | ||| |   4|----------            | >< |  |
   6]--O    |---/    3]--O|]  | ----    3|----------            | >< |  |
   7]--O     |       2]--O|   ||        2|----------            | >< |  |
   8]--O     |       1]--O|___||        1|----------            |####|  |
    #--------#        #--------#         #------------------------------#


                BOTTOM                                  SIDE
    #------------------------------#     #------------------------------#
    |                              |     |                              |
    |                              |     |                              |
   |||||||||||||,,,,,,,,,          |     |                              |
   ||||||||||||||||||||||          |     |                              |
   |||||||||||||`````````          |     |                              |
    |                              |     |                              |
    |                              |     |                              |
    #------------------------------#     #------------------------------||
                                                             ///|||||||||||
                                                      ////////

This will help you understand the orientation of the RJ45 plug and pins
as well as their associated numbers.

Normally, when you wish to connect an ethernet card to a hub, you map out
the conductors to their corresponding locations. (That is, you make sure that
the Same color wire that is on the (1) position at one end is also in the (1)
position on the other end.)   :



This ASCII-Art shows how pinout and numbers for each end of a "patch cable"
should look. A "patch cable" is what is used to connect a computer to a
switch, repeater, hub, etc.
 
     (TOP VIEW POINTING LEFT)                 (TOP VIEW POINTING RIGHT)
  ------------------------------           ------------------------------
8|----------                    |         |                    ----------|1
7|----------                    |WIRE-WIRE|                    ----------|2
6|----------                    |WIRE-WIRE|                    ----------|3
5|----------                    |WIRE-WIRE|                    ----------|4
4|----------                    |WIRE-WIRE|                    ----------|5
3|----------                    |WIRE-WIRE|                    ----------|6
2|----------                    |WIRE-WIRE|                    ----------|7
1|----------                    |         |                    ----------|8
  ------------------------------           ------------------------------
(Notice that the number assignments is effectively the same for both ends.
Rotate the above pictures clockwise 180 degrees, and you should see, the "1"
is at the bottom on the left side RJ45 jack, and "8" is still at the bottom
of the RJ45 jack on the right.)

If you wanted to connect two computers together without using a hub, or
connect two hubs or switches together without an "uplink" port, then you
would have to make (or buy) a special kind of 10T cable sometimes called
"uplink cable" or "null connector" or most commonly, "Crossover Cable"
Normally:
1)TX (Transmission) pair on one end are mapped to the TX pair on the other end.
2)RX (Receive) pair on one end are mapped to the RX pair on the other end.

If you only have 2 computers, and you do not have a hub, or you wish to test
a network between two computers without a hub, a slight modification would
be needed to the above ASCII art diagram. Mainly, the RX pair at one end would
need to be switched with the TX pair at the other end, AND the the TX pair
would need to be switches with the RX.

This would effectively make the TX lines for one machine, the RX lines for
the other, and the TX of the other machine into the RX of the former machine.

So an example of what this would look like, if you were to make a 10T connector
that was to be used for connecting two machines together without a hub:

     (TOP VIEW POINTING LEFT)                 (TOP VIEW POINTING RIGHT)
  ------------------------------           ------------------------------
8|----------                    |         |                    ----------|3
7|----------                    |WIRE-WIRE|                    ----------|6
6|----------                    |WIRE-WIRE|                    ----------|1
5|----------                    |WIRE-WIRE|                    ----------|4
4|----------                    |WIRE-WIRE|                    ----------|5
3|----------                    |WIRE-WIRE|                    ----------|2
2|----------                    |WIRE-WIRE|                    ----------|7
1|----------                    |         |                    ----------|8
  ------------------------------           ------------------------------

(NOTE: this is also the method for connecting two hubs together.)

As a suggestion, in most cases, you do not need to have *really* long
10T connectors that have the RX and TX switched for connecting two machines
together without a hub. It is advisable to make one or two *really* short
10T Male to Female connectors to insert or connect to a normal 10T line.
The advantages are obvious.
1) You do not need to label each long connector as being a hub-to-hub connector
or as a test-connector-without-a-hub.
2) Moving around 10T connectors that are in a wall can be time consuming
(remove the wall and re-wire? AK!)
3) Moving a short little connector to wherever it is needed is very easy,
and it fits inside most tool boxes.
4) No confusion as to which 10T connector is HUB-to-HUB/COMPUTER-COMPUTER
and COMPUTER-to-HUB.

This next part can instruct you on how to make a 10T short connector that
is Male on one end and Female on the other.

First, it is possible to buy 10T Female connectors, but I have not found any
that can be crimped with the standard crimping tools. (I am sure that they
exist, but I was unable to find them.) Something that is more common is
something you can pick up at most business telephone supply stores, or
electronic supply stores. It of course is a 8-conductor RJ-45 Telephone
Female to Female connector.

BUT WAIT! There s a real problem here! These RJ45 Female to Female connectors
are linear connections. They map out their conductors directly:

1----------1
2----------2
3----------3
4----------4
5----------5
6----------6
7----------7
8----------8

You remember from above that as the orientation of the 10T connector then,
that the numbers appear to invert (this is not to say that the starting point
for the (1) changes, it is to say that, if you marked the (1) side with an
ink pen, and then rotated the JACK 180 degrees, the mark will now bee visible
on the opposite side.)

*SO* (this is going to be an interesting wire by the way),
if you are going to make a short 10T connector that has M to F that can be
added to a regular Computer-to-hub-10T-connector, you would not only need to
switch the RX and TX pairs *BUT* you would also have to invert the order.
Here is what it would look like:

(WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!)
(WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!)
(WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!)

THIS IS NOT THE NORMAL CONNECTOR, PLEASE READ THE ABOVE INFORMATION.
(don't just rely on "looking at the pictures!"                        ;)

(WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!)

     (TOP VIEW POINTING LEFT)                 (TOP VIEW POINTING RIGHT)
  ------------------------------           ------------------------------
8|----------                    |         |                    ----------|8
7|----------                    |WIRE-WIRE|                    ----------|7
6|----------                    |WIRE-WIRE|                    ----------|2
5|----------                    |WIRE-WIRE|                    ----------|5
4|----------                    |WIRE-WIRE|                    ----------|4
3|----------                    |WIRE-WIRE|                    ----------|1
2|----------                    |WIRE-WIRE|                    ----------|6
1|----------                    |         |                    ----------|3
  ------------------------------           ------------------------------
(WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!WARNING!!!)

Now you can attach the Female to Female 8 conductor (4 lines/8 wires/4 pair)
plug to the right hand side and label this plug something like,
"10T NULL connector for HUB-to-HUB or COMPUTER-to-COMPUTER connections"

Then when you want to have a long TEST cable or HUB-to-HUB cable, just
connect the Male end of the long cable to the other end of a female plug,
and you have what you want. :)

=-----=#=------------------------------------------------------------------=
 SHORT                    LONG 10T COMPUTER-TO-HUB CONNECTOR
10T PLUG                  This could be running through a wall,
THAT WE                   or you could have a number of these
JUST MADE                 just lying around.




Extra Info (academic)
Numbered lines and their usage:
1=TX+
2=TX-
3=RX+
4=N/A
5=N/A
6=RX-
7=N/A
8=N/A

In standard 10BaseT, only conductors 1, 2, 4 and 6 are used. Why are the 
conductors 4 and 5 skipped? Analog telephones in the US and elsewhere use
pins 4 and 5 for the first phone line.

A single RJ45 jack can allow for up to 8 analog phone lines:
Line #                      Pins:
1                           4 and 5
2                           3 and 6
3                           2 and 7
4                           1 and 8

So to the previous question, why are conductors 4 and 5 skipped for 10BaseT
EtherNet?
When an analog telephone is in normal operation, the amount of current and
voltage is quite small, but when a telephone is directed to ring, there is
a change in current and voltage which is sufficient to DESTROY many network
interface cards.
Digital phone lines and multi-line phones which used RJ45 jacks have the
potential to DESTROY your network interface card if you try to use them
with your NIC.