Is it the input or the output? Cut the fault in half
A signal starts at a field device and ends at an actuator, passing through the input card, the CPU, and the output card. Read the tags and the terminals, and you cut the problem in half before you touch a tool.
A faultline101 training module · the skill behind six hands-on scenarios
Input or output? Splitting the fault in half
Compare the field device to the PLC input tag, and the output tag to the actuator — and you cut a machine fault down to one link before you touch a tool.
The idea
Every automated function is a chain: a field device feeds an input card, the CPU runs the logic, an output card drives an actuator. When something won't work, the fault is in exactly one link — and the PLC lets you find which one in seconds.
The move is a half-split. On the input side, compare the field device to the PLC's input tag: agree, and the input is fine. On the output side, compare the PLC's output tag to the actuator: agree, and the logic did its job. The disagreement is your fault.
This is the difference between 'the machine's broken' and 'the fault is between the output card and the valve.' One is a shrug; the other is a repair. The tag values tell you which world to walk into — field, wiring, or logic — so you never tear into the wrong one.
Six faults you'll work in this module
You don't learn this by reading it — you learn it by doing it on six different faults, each drilling the same skill until it's instinct:
- Good device, blind PLC. Good device, dead tag — the fault is the input side, not the field.
- The PLC isn't lying. Terminal and tag agreed; that pointed straight at the device.
- The command that never arrives. Command and card voltage present, dead at the device — it's downstream wiring.
- Commanded, but no volts. The logic did its job; the output hardware didn't.
- The tag that lies. A force makes a tag lie — clear forces before you trust any I/O.
- Split the whole chain, cold. From 'the station's broken' to 'replace this output point' in minutes.
The mistakes it kills
- Assuming the field device is bad without checking whether the PLC even reads it correctly
- Tearing into the logic when the output tag and card voltage prove the code already did its job
- Forgetting that a forced I/O point can make a tag lie about the real device
Why it matters: Splitting a fault into input-side, logic, or output-side in the first two minutes is what stops a four-hour teardown of a machine that only had a loose wire.
Reading about it is one thing. Doing it under pressure is another.
faultline101 drops you into a running machine with the real evidence in front of you, and the readings respond to your choices — exactly like a real callout, on equipment you can't break. This is one of 35 modules and 210 scenarios in the trainer.
Works with no signal. Install it on your phone and the whole course — 35 modules, 210 faults — comes with you into the plant.
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