مدونة الريس: Reverse Engineering 101 - All roads lead to Rome: Finding the return path

When examining any electrical system, even a simple two wire chord, it is essential to identify the return path of the electric current, which is the circuit ground in case of DC or neutral in case of AC. To do so with circuit boards without a schematic or a system seen for the first time there are few tips the reader can follow such as:

Look for standard connectors such as USB, RS-232/485, barrel jack connector, DC power input connector such as terminal blocks...etc., all these connectors have a “ground” pin noted in the connector’s standard that can be identified easily, and usually it is connected to the circuit board common ground plane/traces.

Look for power management components such as voltage regulators, LDOs, DC-DC converters and their controllers, battery BMS, full-wave rectifiers, decoupling capacitors, from the datasheet you can identify the ground pin and using the DMM’s continuity tester you can trace it to the circuit board’s ground.
From the IC datasheet try to identify the ground pin in any of the integrated circuits on the circuit board, ICs such as processors, logic circuits, timing, silicon oscillators have a ground pin connect to the board’s ground.
Look for LED indicators, as they can be directly connected to ground but that varies between different designs (especially when current-sink LED driver is used, the LED isn’t connected directly to ground).
Avoid discrete components to some extent as some of them might not have a direct connection to the signal return path such as inductors and power MOSFETs in some DC-DC converter designs.
Tracing ground through analog components that operates from a dual power supply (+/- power supply) must be done carefully to avoid confusing the board’s ground with the negative power supply.

In the picture below of D-Link DIR-615 WIFI router main board, you can easily trace the ground plane through the barrel power connector, the copper pour on the board's top layer, or the ground pin of any of the integrated circuits show, and maybe through the indicator LEDs depending on the their connection to corresponding GPIO pins as current-source or current sink.