Led's 8 Wires:
Not a mistype. The problem with testing each wire individually is that if you had 7 individually addressable LED's, then you would need an eight return or common wire. In the case of testing 8 wires you would need a ninth wire. You could use a domestic earth but its not really practical, and also if the cable was shorting to earth anyway it would be no good anyway. The solution had me thinking for a while, but since this is a logic circuit, there are only two conditions, logic high or zero. As the 4017 outputs are either high or low, any output can provide a common return path for a LED. So LED's 1 - 3 use the 4th output of the 4017, which will be zero, and the 4th LED is wired with reverse polarity. On the 4th pulse, output 4 is high, output 3 is low and so the LED will light. If the common return wire is open circuit then LEDs 1-4 will not light. A similar situation occurs with outputs 5 to 8. The common wire in can be taken from any output terminal from the 4017, but the same rule would still apply. The ability to test all wires quickly outweighs this small disadvantage. If a cable of just 4 or 6 wires is tested then it must use the wires with LED's numbered 1 to 4 or 1 to 6, which is why the LED's are numbered that way. Testing:
With a good cable and all wires connected then LED 1 will light at both cable ends, followed in sequence by LED 2 ,3, 4 etc to LED 8, the sequence then repeating. If a 4 wire cable is used, it must be connected to use the common return wire as described in the preceeding paragraph. The sequence would be LED 1,2,3,4 repeating with a delay as the 4 unused outputs are stepped through. To check for earth contact faults, the probe labeled "to earth connection" would be physically connected to a local earth. A wire that is earthing will dim or extinguish the LED's at both ends of the cable. An LED not lighting at the receiver, indicates a broken or open circuit. If two wires are short circuit, example 3 and 4 then at the receiver the sequence would be 1, 2, 34, 43, 5, 6, 7, 8. A reversal would be indicated by an out of pattern sequence of LED's. Here's an example, the probe is connected to an earth at the transmitter, the cable is very faulty, wire 1 is OK, 2 is earthing, 3 and 5 are reversed 4 is OK, 6 is open circuit and 7 and 8 are short circuit. See below.