Copper to code: read the I/O handoff
This is the seam between the two tracks. A field device wires to a terminal, the terminal lands on an I/O point, and the I/O point becomes a PLC tag. Read the I/O drawing and you can walk a signal from copper to code and find exactly where it breaks.
A faultline101 training module · the skill behind six hands-on scenarios
Where the print meets the PLC: I/O cards, terminal blocks & the handoff
Field device to terminal to I/O point to PLC tag — read the I/O drawing and you can walk a signal from copper to code and find exactly where it breaks.
The idea
Every input and output crosses a seam: the physical wiring world and the PLC tag world meet at the I/O card. A field device lands on a terminal, the terminal wires to a specific I/O point, and that point maps to a PLC tag. The I/O wiring drawing is the page that shows this handoff — device, terminal, point, and tag, all lined up.
Reading it lets you localize a fault to one side of the seam. Meter the field device at its terminal, then read the PLC tag: if the device is present at the terminal but the tag is 0, the break is between the terminal and the tag — the wiring to the point, the point itself, or its addressing. If both agree, the seam is fine and the fault is elsewhere.
Two things live on this drawing that trip people up: field power (many I/O points need a supplied 24 V to see anything, and if that's gone the point is blind), and the sinking/sourcing convention (which common the device returns to). Get those from the drawing and the handoff reads cleanly, every time.
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:
- Copper to code. Good terminal, dead tag — the fault is the input-side seam.
- The blind point. The drawing showed the field-power dependency the point needs.
- Right device, wrong common. A good device reads nothing if the common is wrong.
- Bridge the two worlds. The handoff drawing joined the electrical and PLC worlds.
- Landed on the wrong point. The handoff was physically wrong versus the drawing.
- Cold: field to tag. Tag to point to terminal to device — seam crossed cleanly.
The mistakes it kills
- Not comparing the field terminal to the PLC tag, so you can't tell which side of the seam the fault is on
- Forgetting the I/O point needs field power to see any input
- Ignoring sinking vs sourcing, so a correctly-working device wired to the wrong common reads nothing
Why it matters: The I/O handoff is where electrical troubleshooting and PLC troubleshooting meet — reading it is what lets one technician follow a fault all the way from the field device to the tag without handing off.
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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