When a machine won't start, the amateur swaps parts at random; the pro follows the signal chain and finds the fault in minutes. It is the most valuable skill in the trade and the hardest to teach from a book, so this lesson gives you a diagnostic simulator. A machine breaks, and you probe the chain link by link to isolate the cause. Once you have the method, you can work through any fault the same way.
Every machine action is a chain of signals: a button → an input → logic → an output → a relay → a motor. When the motor doesn't run, exactly one link is broken. The whole skill is bisection. Instead of guessing, you probe the middle of the chain, learn which half the fault is in, and repeat. Three probes are enough to find the fault in an eight-link chain.
🧭 The golden question at every link: "Is the signal correct here?" If yes, the fault is downstream. If no, it's upstream. You never swap a part until you've proven that link is the break.
Each scenario breaks the chain somewhere. Click a link to probe it and the simulator tells you whether the signal is correct there. Follow it from the button to the motor (or bisect smartly) until you isolate the one broken link, then call it. The fewer probes you need, the better the technician.
👁️ Online monitoring
Go online and watch the live bits. Input ON but logic not acting? Logic bug. Logic commanding but output dead? Field/wiring. This one check splits the whole problem in two.
🚦 Forcing, carefully
Force an output ON to test the field side (does the valve move?), or force an input to test the logic. Forcing gets you answers fast, but it overrides your logic and every software interlock, and it can start a machine moving. Never use a force to defeat a safety function. Force deliberately and remove every force before you leave.
📋 Diagnostic buffer
The controller's black box: timestamped faults, mode changes and module errors. On an intermittent or overnight fault, read this first; it often hands you the answer.
Commissioning is troubleshooting in advance: you prove each layer works before you stack the next one on top. The safe order is:
1 · Power & I/O check: verify supplies, then check every input toggles and every output fires (often with forcing), before any automatic motion.
2 · Dry run: run the sequence with no product/at low speed, guards engaged and a finger on the E-stop, so you catch logic errors safely.
3 · Wet run & tune: introduce real product, tune speeds/PID, handle the edge cases. Then ramp to rate.
4 · Sign-off (SAT): prove it meets spec against the acceptance test, with safety validated. Document and hand over.