I have given this idea to an instructabler about his cable tester, I take chance to talk more details. This is a simple tools help to test the correctness of an ethernet cable, especially those crimped by yourself, just use a simple ohm meter to check the continuity of cable pairs.
Step 1: RJ45 Connector Boxes
The overall structure as shown, each cable pair is serial by a diode, why diode? will be mention later. This tools only contain two identical RJ45 connector box, on one box terminal each cable pair by a diode, the other box has nothing to do but drilled some small holes to let the testing leads can be passed through.
Step 2: Why Diodes?
Diodes are one kind of semi conductor, by its mean, current can be passed one way only. The symbol of diode tells the story, current can pass through form positive side to negative side which has the vertical bar marked on the diode. For traditional Japanese style meter, the black lead holds the positive polarity of the internal battery in the ohmic range. Such connection called the forward biased, the resistance measured only several ohms in Rx1 range, but will not be zero, while the reverse measurement gives no reading.
When you serial a cable with a diode, you measure the cable on the other end same as measuring a diode, it gives you the possibilities of one way conduct, the other way won't, you can determine the cable pairs are properly terminated, without crossed pairs, reserved pairs, shorted pairs and opened pairs.
Step 3: How It Works?
Just plug both ends of the cable into the connector boxes, set the meter in ohmic range start from Rx1 range, insert the testing leads to touch the terminal box and observe the needle movement, reverse the testing leads and test again, test all pairs if required.