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I have a NS-468 cable tester for testing cables. This works fine but if I connect just one end of a RJ45 cable to a network, I only get 4 lights. This seems reasonable as I suppose 4 pins are for sending and 4 pins for receiving (sorry, I'm not a network person!). But I get different sequences depending on what network I connect to (e.g. 1-2-3-6 or 1-2-5-6). Why is this?

Also, I'm trying to troubleshoot a connection which is only sending packets but not receiving. On this connection, it just cycles through like a cable test (1-2-3-4-5-6-7-8). Does this indicate a problem?

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  • That tester seems to do not much more than give you a wire map, which is only one of the tests necessary. See this answer for the basic tests that are necessary to check. A real cable tester costs a lot of money.
    – Ron Maupin ♦
    Commented Oct 30, 2018 at 19:01
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    Hello Robbie and Welcome. I'm assuming it's ethernet you're using, in which case it's almost impossible to have hardware fault which only transmits, because it won't transmit anything useful without successfully receiving ARP replies. On the other hand it's easy to make IP routes only work in one direction; suggesting misconfiguration. You may well find your cable is faulty, but as Ron says, you'll only see gross faults with your cable mapper. It's possible that 1-2-3-6 indicates two-pair wiring, which might be ok. 1-2-5-6 indicates a gross fault.
    – jonathanjo
    Commented Oct 30, 2018 at 19:29

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According to the instructions for the NS-468, it is designed and engineered to use the master unit at one end of the cable and the remote unit at the other end of the cable.

Based on the instructions, this is a basic continuity tester which can only verify continuity from one end of a conductor to the other end. It can validate that the conductors are terminated in a certain order and that electrons can flow across the conductor. It cannot determine if the terminations or cable is faulty in any other way important to networking.

This type of test tool does not contain a NIC (network interface card) nor is it meant to be used when connected to the network. Trying to understand the output from a test tool being used for something it is not designed/engineered to do will likely not produce results that are meaningful in any way.

If you want a test tool meant to give results when connected to the network, you would need to get a tool designed for doing so.

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This type of cable tester is intended for bare cables only. Connecting it to a live network can damage the network component on the other side or (more likely) the tester itself, esp. on a PoE link.

When connected to a normal network port (again, not recommended) you measure the DC resistance/continuity across each pair that's connected by the magnetics/transformer within the port. Its DC resistance is quite low, so the tester may show continuity for the 1-2 and 3-6 pairs on a 10BASE-T or 100BASE-TX port (and likely 1-2, 3-6, 4-5, and 7-8 for 1000BASE-T). (1-2 and 5-6 indicates a miswiring fault.)

Many 10/100 ports also connect the spare pairs (4-5, 7-8) to ground, so the tester might show those as shorted.

Instead, use the tester as intended, read the manual and either connect one piece of the tester to each side of a cable (obviously disconnected from everything else), or plug both ends into the active piece. The tester checks for proper continuity, pairing, and shorts. If a problem is indicated it will impact the network connection.

However, as YLearn has already pointed out, those testers can't guarantee for the actual network function, only the lack of basic cable functions. There's much more to proper network connection than continuity and absence of shorts. The link needs to be able to properly transmit a signal of at least 100 MHz (Cat-5 for 100/1000 Mbit/s) with very little crosstalk. A proper network tester checks all parameters of a cable (see Ron's link in his comment) and starts at $600 or so, depending on the cable grades it supports.

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