Journeyman refresher Refresher 7 of 10 · about 9 min
Rusty on PLCs?
What you’ll learn
- The scan cycle and what the input and output images are.
- XIC and XIO, and the NC stop-button trap that catches everyone once.
- Latches, timers and counters in one line each.
- Troubleshooting from the I/O lights, and forcing without hurting anyone.
Before this: Intro to PLCs
Quick reference
| Item | What it does |
|---|---|
| Scan | Read inputs → solve program top to bottom → write outputs → housekeeping → repeat. |
| XIC | True when the bit is 1. |
| XIO | True when the bit is 0. |
| OTE | Bit follows the rung, like a relay coil. |
| OTL / OTU | Latch sets the bit and leaves it; unlatch clears it. Survives rung going false. |
| TON / TOF | On-delay / off-delay timer; the done bit changes after the preset. |
| CTU / CTD | Count up / down on each false-to-true transition; done bit at the preset. |
| Force | Overrides a real input or output. Stays until removed. |
Worked: reading the lights on a dead conveyor
- Operator presses Start; conveyor does not run. Stop input LED is ON at rest — correct for an NC stop.
- Start input LED comes ON when pressed — the button and its wiring are good.
- Motor output LED stays OFF. The program is not commanding it, so look at the rung: a permissive (guard switch, e-stop relay) is false. The guard input LED is off.
- At the guard switch: meter reads the input voltage across its contacts — it is open. Adjust the actuator, the LED comes on, the rung goes true, the conveyor runs. Three lights and one reading.
On the job
Tips
- Input LED tells you the field; output LED tells you the program. Decide which side of the card the fault is on before you open the laptop.
- Hard-wired safety (e-stops, guard relays) should stop the machine without the PLC. Check it is there.
- Log every force you apply and remove it before you leave.
Common mistakes
- XIO on an NC stop button — the motor runs only while Stop is held.
- Using the same output address on two rungs; the last one solved wins.
- Forcing an input to “see what happens” with people near the machine.
- Assuming a latched bit reset itself after a power failure.
Check yourself
-
1.A Stop pushbutton is wired as a normally closed contact to a PLC input. Which instruction do you use for it in the Stop position of a start/stop rung?
The NC button holds the input ON while nobody presses it, so the rung must look for the input being ON: XIC. Pressing Stop turns the input OFF and breaks the rung. Using XIO here is the classic trap — the motor would only run while Stop is held.
-
2.Why is Stop wired as a normally closed contact in the first place?
Fail-safe: a cut wire looks the same as a pressed Stop, so the machine stops. An NO stop with a cut wire would look like “not pressed” and the machine could never be stopped from that button.
-
3.In the scan cycle, when does the PLC read the physical inputs?
A typical scan: read inputs into the image table, solve the program top to bottom, write the output image to the outputs, housekeeping, repeat. An input that flickers on and off within one scan can be missed entirely.
-
4.The input LED for a limit switch is ON but the rung that uses it stays false. Where is the problem most likely?
The LED says the input card sees the signal. If the logic does not respond, look at the address and instruction in the program. If the LED were OFF with the switch made, you would look at the field wiring instead.
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5.What is the main hazard of forcing an input or output in a running PLC?
A force makes the PLC ignore reality at that point. It can defeat interlocks and start motion without the normal conditions. Forces persist until removed, which is why sites control them and log them.
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6.An output rung is true and the output LED is ON, but the motor starter does not pull in. What next?
Once the output LED is on, the program has done its job. From there it is ordinary field-circuit troubleshooting with a meter.
Questions people ask
Is XIC the same as a normally open contact?
It behaves like one on the screen — it passes power when its bit is 1. But it is an instruction examining a bit, not a physical contact. Whether the field device is NO or NC decides what the bit is at rest, and that decides which instruction you need.
What is the difference between OTE and a latch (OTL/OTU)?
An OTE is on only while its rung is true, like a relay coil. A latch turns its bit on and leaves it on until an unlatch rung turns it off — even through a power cycle on many PLCs. Latches are why a machine can come back after a power failure in a state nobody expected.
Can I troubleshoot a PLC without the software?
A long way, yes. The input and output LEDs tell you what the PLC sees and what it is commanding. Field device ON but input LED OFF: field wiring or input card. Output LED ON but load dead: output circuit. Only when inputs look right and outputs look wrong do you need the program.
Related
- Intro to PLCs — the full lesson
- Troubleshooting industrial controls
- Start/stop and the seal-in (lab) — the hard-wired version
- Reading a ladder diagram
- The troubleshooting method
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