Which Pin Maps to Which Wire?#

Cable tracing, connector pinouts, and verifying wiring against a schematic. When staring at an unfamiliar harness, a hand-wired prototype, or a cable that “someone else made,” the task is mapping what’s actually connected — not what the label says.

Continuity-Based Wire Tracing#

Disconnect the cable at both ends. Pick a pin on one end, touch one probe to it, then touch the other probe to each pin on the far end until a beep indicates connection. Record the mapping and repeat for every pin.

For multi-pin connectors, a systematic approach saves time:

1. Number or label every pin on both ends if not already marked
2. Short one pin on the far end to a known reference (shell, or pin 1)
3. From the near end, find which pin beeps to that reference
4. Move the short to the next far-end pin and repeat

This reveals the actual pin-to-pin wiring, any pins shorted together (unintentional bridges), and any pins that are open (broken wire).

Resistance-Based Cable Quality#

Beyond pass/fail continuity, resistance measurement characterizes wire health:

1. At the far end, short together the test pin and a return pin (creating a loop)
2. At the near end, measure resistance across those two pins
3. Divide by 2 for one-way resistance
4. Compare wires of the same gauge and length — they should read similarly

Expected resistances:

Verifying Wiring Against Schematic#

With schematic visible and circuit powered off:

1. For each connection shown, verify continuity between the two points
2. Mark each verified connection on the schematic
3. Also check for unintended connections — especially between adjacent pins or nets that run close together

Tips#

• Disconnect cables at both ends before tracing — active electronics in the path give misleading results
• Null DMM lead resistance for short cables where lead resistance matters
• Map the shield separately on shielded cables — it connects to specific pins (usually shell/ground)
• Know what wiring pattern to expect — some cables intentionally cross over (null modem, crossover Ethernet)

Caveats#

• Active cables (USB-C, HDMI, some Ethernet) have electronics inside — continuity testing shows internal circuit resistance, not direct wire paths
• In-circuit continuity can show connections going through components rather than the intended wire — trace carefully if results seem wrong
• Schematics don’t always show every connection (power pins sometimes implied, grounds shown separately) — check conventions before declaring “extra” connections as faults
• Corroded connections add resistance at the connector, not in the wire — high reading on one conductor suggests crimp or solder fault
• Temperature affects resistance — copper has positive tempco; a cable in a hot environment reads higher

In Practice#

• One conductor reading significantly higher resistance than others of the same gauge indicates damage or poor termination
• Pin that beeps to multiple far-end pins indicates an unintentional short (bridge) in the cable or connector
• Pin that doesn’t beep to any far-end pin indicates an open wire — break is in the cable or at a termination
• Wiring that doesn’t match expected pattern could be a fault or could be correct for that cable type (crossover, null modem) — verify expectations first
• Continuity that appears through a component path rather than direct wire suggests the measurement is seeing the circuit, not the intended connection