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Analog & I/O·6 min read·

Wiring I/O without creating problems for later

A short list of practices that make the difference between a panel that is easy to fault-find and one that is not.

Short answer

Keep signal and power segregated, ground shields at one end only, use interposing relays for anything inductive, and label both ends of every wire with the same identifier used in the program. The single highest-value habit is making the label on the wire, the terminal, the drawing and the tag name identical.

Sinking and sourcingSINKING (NPN)INPUT CARD+current flows inSOURCING (PNP)INPUT CARD0Vcurrent flows out

Most of what makes a panel easy to work on is decided at wiring time by somebody who will never see the fault.

Segregate

Signal wiring and power wiring in separate trunking, crossing at right angles where they must cross. A VFD output cable running parallel to an analog signal induces noise that looks exactly like a faulty sensor.

Ground shields at one end

At the panel. Grounding both ends lets current flow along the shield when the two ground points differ in potential, which injects the noise the shield exists to stop.

Interpose anything inductive

Contactor coils and solenoids produce a spike when they release. An interposing relay takes that abuse instead of the output card, and costs a fraction of the card.

Label consistently, everywhere

The wire number, the terminal number, the drawing reference and the PLC tag should be recognisably the same thing.

This is the highest-value habit in the list. Every minute spent matching a wire to a tag during a fault is a minute the line is down, and it is entirely avoidable at build time.

Leave spare

Spare cores in every multicore, spare terminals on every rail, and spare points on every card. The change nobody planned for always arrives, and it arrives at the worst moment.

Common questions

Should cable shields be grounded at both ends?
Normally at one end only, at the control panel. Grounding both ends creates a path for current to flow through the shield when the two ground points sit at slightly different potentials, which injects exactly the noise the shield was there to prevent.
Do I need interposing relays for PLC outputs?
For anything inductive or drawing significant current, yes. Contactor coils, solenoids and motor starters generate voltage spikes when they release, and driving them directly shortens output card life. A relay is cheaper to replace than a card and isolates the fault.

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