Testing a network cable means proving continuity on all eight conductors, proving they land on the correct pins at both ends, proving no two touch, and then proving the cable carries real traffic without errors. A meter alone can do the first three with 36 checks for one cable, but it cannot detect a split pair, which passes continuity and fails at gigabit speed. The final proof is a file transfer between two machines with the switch port’s error counters read afterwards.
Part of Networking
Verifying a copper network cable with a tester, with a multimeter, and under real traffic, and knowing what each method can and cannot detect.
Continuity Is Necessary And Is Not Enough
Twisted Pair Wiring lists three things a tester must check: continuity (all eight conductors connected through), the wiring map (each conductor on the right pin at both ends) and, beyond a basic tester, the pairing and signal quality. It also states the consequence: a split pair passes the continuity test and still fails at gigabit speed. Ethernet Basics adds that most cable problems are connector problems: misaligned conductors, incomplete crimps, or a broken wire at the plug.
So there are three levels of test, and each catches faults the one below cannot:
- Continuity and wiring map — opens, shorts, crossed or swapped conductors
- Pair and signal tests — split pairs, crosstalk, attenuation (a certification instrument; see the end of the twisted-pair article)
- Traffic test — a real transfer at the speed the link is meant to run
What You Need
- A simple cable tester — a main unit and a remote that shows each pin lighting in order; a basic one is enough for level 1 (Network Hardware)
- Or a multimeter on its continuity range — see Continuity Testing for method, and note that the test is made on a cable that is not connected to any powered device
- Two computers and a switch — for the traffic test
- Access to the switch’s port statistics — error counts show a marginal cable that passes a simple tester (Star Topology)
Method: Test A Cable Three Ways
Step 1: Wiring map with a tester Plug each end into the main unit and remote. The lights should step through pins 1 to 8 in order on both. A skipped pin is an open, two lights at once typically mean a short, and out-of-order lights are miswires.
Step 2: The same test with a meter if there is no tester If both ends reach the same bench (a patch cable under test), probe conductor to conductor directly. For a fixed run, use a long test lead to the far end or a second person calling out readings; a pair of tester units saves most of the work. For an open: check each conductor pin to the same pin at the other end. For a short: check each conductor against every other conductor at one end, where there should be no continuity. Count the checks in the worked example below.
Step 3: Traffic test Connect two machines through a switch with the cable in the path. Start a large file transfer (File Transfer) and let it run. Afterwards read the port’s CRC and frame check sequence error counts (Twisted Pair Wiring): a marginal cable shows errors, and a good one shows none. Also note the negotiated speed; a link that comes up slower than both ends support suggests a bad pair (Twisted Pair Wiring).
Step 4: Record and label Write the result on the label and in the cable record.
Worked Example: How Many Checks Does A Meter Need
An RJ45 cable has 8 conductors.
Opens. One check per conductor end to end: 8 checks.
Shorts. Every pair of conductors that must not touch is a combination of 2 out of 8: 8 x 7 / 2 = 28 checks.
Total. 8 + 28 = 36 checks for one cable. This is why a tester that does it in one pass is worth building or salvaging before there are dozens of cables. Even after all 36 pass, the pairing is unproven: a split pair has all eight conductors connected through and no shorts, so it gives exactly the same 36 results as a good cable.
Common Mistakes
- Treating a passing continuity test as a pass — it proves nothing about pairing or signal quality
- Testing a cable that is still plugged into a switch — keep the cable disconnected from powered equipment for the meter test
- Not rechecking after a move — flexing a cable can break a conductor at the plug; flex each connector while watching the link light (Connectors)
Safety
Never test live wiring by touch
A multimeter continuity test is made on a cable that is disconnected at both ends. Salvaged cable that ran in a wall with mains wiring may have been energised, so prove it dead first (Electrical Safety).
What’s Next
- Learn next: Physical Layer — why attenuation and crosstalk set the limits being tested
- Combine with: Cable Crimping — fixing the faults that this finds