WiremanWireman · PLC input not seen: troubleshooting for electricians · 2023 NEC ·
PLC input not seen
Voltage at the input terminal, the input LED, then the program.
Before you start
How much electrical experience do you have?
This sets how much detail each step shows. You can change it any time.
Showing detail
How it works
A PLC input card watches for voltage across each input and its common. Most DC inputs are 24 V DC. A sinking input expects the field device to supply +24 V to it (so it pairs with a sourcing or PNP sensor), and a sourcing input expects the field device to pull it to 0 V (so it pairs with a sinking or NPN sensor). Either way, the input LED shows what the card sees, and the program decides what to do with it. That gives three places to check: the field wiring, the card, and the logic.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When the device should be on, is that input’s LED lit on the card?
- LED on, program doesn’t reactOnline with the PLC, does the input’s bit change state in the program?
- Bit doesn’t change, or it’s forcedAddress mismatch, force, or PLC not in Run
- Bit changes, but the output doesn’t followThe program logic is holding things up
- LED offTest: Measure across the input and its commonExpect: About 24 V DC across the input when the device is on, near 0 V when it’s off. The supply reads about 24 V DC. Check the card’s on and off thresholds in its spec.
- No 24 V supplyNo 24 V DC supply
- Never changes when device operatesField device or wiring
- 24 V across the input, LED still offIs the card’s common wired to the correct side of the supply for this card and sensor type?
- Mismatched or common not landedSinking/sourcing mismatch or missing common
- Wiring matches the cardFailed input point or card
- LED stays on all the timeTest: Measure the input with the device offExpect: Near 0 V with the device off, and the LED out.
- Voltage present with device offLeakage or a stuck field device
- LED stays on with the wire liftedFailed input point or card
- LED on, program doesn’t reactOnline with the PLC, does the input’s bit change state in the program?