Developing Fundamental PLC Wiring Diagrams & Ladder Logic Programs

Electromagnetic Control Relays

A control relay is the simplest electromagnetic device on the floor: a small electromagnet that pulls a set of contacts closed (or open) when its coil is energised. Apply 24 V DC to the coil and the contacts switch — that’s it. Drop the voltage and a spring snaps them back.

Relays solve one problem brilliantly: signal isolation. The PLC’s tiny output transistor cannot directly switch the 30 A motor in your machine. Instead, the PLC switches a small current through the relay coil, and the relay’s much beefier contacts switch the load. Same idea your finger uses when pressing a light switch — your finger doesn’t carry the current, the switch does.

Two terms to lock in:

• Coil — the electromagnet itself. Drawn as a circle with the letters CR (Control Relay) plus a number. Energising the coil “operates” the relay.
• Contacts — the actual switches the coil moves. A single relay can have many contacts. Each is either NO (normally open, closes when energised) or NC (normally closed, opens when energised). The “normal” state is always the de-energised state.

Contactors — Heavy-Duty Relatives of the Relay

A contactor is essentially a relay built for muscle. Same electromagnet-and-contact idea, but the contacts are sized to carry tens or even hundreds of amps. Contactors switch motors, large heaters, lighting banks — anything where a regular relay’s 5–10 A rating would melt.

You’ll recognise a contactor by its size (palm-sized to shoebox-sized), the heavy bus bars it bolts onto, and the much larger main contacts behind a removable arc-suppression cover. Smaller “auxiliary” contacts on the side handle the control-circuit signalling — typically used for the seal-in branch we met in Chapter 5.

Motor Starters and Thermal Overload Protection

A motor starter is a contactor with a thermal overload (OL) relay bolted to its output side. The OL contains a tiny heater coil for each phase, sized so it heats up at the motor’s rated current. If the motor draws too much current for too long — a stuck rotor, a jammed shaft, a failing bearing — the heaters get hot, a bimetallic strip bends, and an OL contact trips, opening to disconnect the contactor’s coil and shut the motor down.

The OL contact is wired normally-closed in the start/stop control circuit, so it must stay closed (no fault) for the motor to run. When it trips, the motor stops, and an operator must manually press the OL reset before the motor can restart. This delay is intentional — the motor needs to cool.

Manually Operated Switches

These are the buttons and switches the human operator touches. They translate a physical action — finger press, hand twist, foot stomp — into an electrical signal.

Pushbuttons

The momentary pushbutton is the workhorse of every control panel. Press it and a contact closes (NO) or opens (NC); release and a spring returns it to rest. NO and NC pushbuttons look almost identical — you read a tiny stamping on the side or check the wiring diagram. START buttons are conventionally NO; STOP buttons are conventionally NC for fail-safe behaviour.

Many real pushbuttons have both NO and NC contacts on the same body, mechanically linked. One press operates both — useful when you need to start one circuit and stop another simultaneously.

Selector Switches

A selector switch stays in whatever position the operator turns it to (no spring return). Two-position (ON/OFF, AUTO/MANUAL) and three-position (HAND/OFF/AUTO) variants are the most common. Internally it’s a multi-pole switch where each rotation position closes a different combination.

Drum Switches

A drum switch is a multi-position rotary that engages many contact patterns at once. The classic application is a forward/reverse motor switch with five positions. Rare on new equipment — most modern designs use individual buttons feeding logic — but you’ll meet drum switches on legacy machinery and low-cost retrofits.

Mechanically Operated Switches

These switches are tripped not by a human, but by the machine itself — by a moving part touching a lever, plunger, or roller.

Limit Switches

The limit switch is the most common mechanically-operated input on any factory floor. A spring-loaded actuator (lever, plunger, or roller) sticks out from the switch body. When a moving part of the machine pushes against it, contacts inside transfer (NO closes, NC opens). Limit switches answer questions like “is the cylinder fully extended?”, “has the conveyor reached the next station?”, “is the door fully closed?”.

Three actuator styles dominate: plunger (push down to operate), roller lever (a wheel on an arm, ideal for cams and tabs), and roller plunger (a small wheel on a vertical plunger). Choose based on the geometry of what’s hitting it.

Foot Switches

A foot switch is a pushbutton you operate with your shoe. Useful when both hands are busy with the workpiece — sewing machines, shears, drill presses, packaging tables. Always wire foot switches as maintained (held while pressed) — never as latching — so the operator can release them in an emergency.

Sensors — A Survey of the Eight Common Types

A sensor detects a physical condition and produces an electrical output (usually a discrete on/off bit, sometimes an analog signal). Choosing the right sensor for a given job is half of automation engineering. Here are the eight you’ll meet most often.

Inductive and Capacitive Proximity

Both look like cylindrical “pencils” that screw into a bracket and have a sensing face. Inductive sensors detect metal only via the eddy currents the target induces in the sensor’s coil. They’re the most rugged and cheapest of the proximity types. Capacitive sensors detect any material (metal, plastic, water, grain) by the change in capacitance between the sensor face and the target. Use capacitive when you need to detect through a non-metal wall — a plastic pipe full of liquid, for example.

Magnetic Reed

A reed switch is a tiny glass-encased contact that closes when a magnetic field passes near it. The classic application is sensing the position of a hydraulic or pneumatic cylinder — a magnet is embedded in the piston, and reed switches mounted on the cylinder body detect the piston at full retract and full extend.

Photoelectric (Light)

A light source shines at a receiver, and the receiver detects either the beam or its reflection. Three common variants:

• Through-beam — separate emitter and receiver. Most reliable, longest range (up to 50 m), but needs both halves wired.
• Retroreflective — emitter and receiver in one body, reflected off a corner-cube reflector. Simpler wiring, shorter range.
• Diffuse — same body, no reflector — looks at light bouncing off the target itself. Easiest to install but range and reliability depend on the target’s colour and finish.

Ultrasonic

Same idea as a bat or a parking sensor — emit a short pulse of high-frequency sound, time the echo. Used for level measurement, distance, and presence detection where vision or proximity sensing isn’t practical (high temperature, transparent targets, dusty atmospheres).

Weight (Load Cell)

A load cell measures force or weight via a strain gauge bonded to a metal flexure. Used on silos, hopper scales, batch weighers, and tension-control rolls. Output is typically a tiny millivolt signal that a dedicated module amplifies into a 4–20 mA value.

Temperature (RTD & Thermocouple)

RTDs and thermocouples measure temperature. RTDs (typically PT100 or PT1000) work by the change in resistance of a precision platinum element as it heats up — accurate, stable, used for process temperatures up to about 600 °C. Thermocouples generate a tiny voltage at the junction of two dissimilar metals — cheap, rugged, and good up to 1200 °C+ for furnace work. Both feed dedicated temperature input modules that handle linearisation and cold-junction compensation.

Flow

Flow switches can be discrete (paddle pushed by flow closes a contact — gives you “flow OK / no flow”) or analog (turbine, vortex, magnetic flow meters output a 4–20 mA signal proportional to flow rate). Discrete flow switches are cheap and fine for “is the cooling water actually flowing?” interlocks; analog flow meters are needed when you have to measure or totalise.

Position

“Position” covers a big family — limit switches at one end, encoders and resolvers at the other. We’ve already met limit switches; encoders give continuous position over many revolutions and feed pulse trains to a high-speed counter input on the PLC. We’ll build a program with an encoder in a later chapter.

Output Control Devices

What can a PLC output drive? Anything that needs an electrical signal, but in practice four kinds of devices cover 95 % of factory applications:

The single rule that protects your PLC’s output card: never let a load draw more current than the output module is rated for. A typical relay output handles 2 A at 120 V AC — plenty for a contactor coil, nowhere near enough for a 5 HP motor. The PLC always commands the motor through an intermediary contactor.

Seal-In, Interlock, and Latching — A Reinforcement

You met all three of these patterns in Chapter 5. Let’s say each one out loud one more time, because if these three patterns are second nature, half of all industrial PLC programming is already in your head.

• Seal-in. A parallel branch around the START button containing an XIC of the output’s own bit. Lets a momentary START button latch the output until something else breaks the rung.
• Interlock. An XIO of one output bit placed in the rung of another output. Prevents two mutually-exclusive outputs from energising at the same time. Forward/reverse motors, up/down hoists, infeed/outfeed conveyors — all use software interlocks. Belt-and-braces them with mechanical contactor interlocks for high-power loads.
• Latching. Persistent state — once set, stays set until explicitly reset. Built either with the seal-in pattern (preferred for motor control because the latch clears on power loss) or with OTL/OTU pairs (for state flags that should survive a power cycle).

Latching Relays — The Hardware Cousin of OTL/OTU

Before PLCs, when a circuit needed to remember its state across power loss, engineers used a latching relay. This is a special two-coil relay: the “set” coil pulls the contacts closed and a tiny mechanical detent holds them there even after the coil is de-energised. The “reset” coil pulls them back open.

In ladder logic, OTL/OTU pairs are the direct software equivalent — except OTL/OTU also lose state on a CPU power loss unless the controller has a battery-backed retentive memory area, which most modern CPUs do.

Converting a Relay Schematic to a Ladder Program

Most factories built before 1990 are still controlled by relay logic that has been “ported” to PLCs over time. The skill of reading an old relay schematic and writing the equivalent ladder rungs is one you’ll use often. The conversion is mechanical once you know the mapping:

Apply this table line by line to any relay schematic and you have your ladder program. Worked program 9 below shows the conversion in action on a classic three-wire motor starter.

Writing Ladder from a Narrative — A Structured Approach

The other half of this skill set is going from prose (“the customer wants the machine to…”) to ladder. Engineers in their first job often freeze at this step because the narrative is open-ended. Use this five-step recipe and the freeze stops happening:

1. List every input device the narrative mentions or implies. Pushbuttons, sensors, limit switches, selector positions. Assign each one an input address. Note whether each is NO or NC.
2. List every output device. Motors, valves, solenoids, lamps, alarms. Assign each an output address. Note any interlocks or mutual exclusions implied.
3. Write the boolean expression for each output. “Motor M1 is on when … AND … OR …”. Don’t worry about ladder syntax yet — get the logic right in plain English first.
4. Translate each expression into a single rung. Series for AND, parallel for OR, XIO for NOT. Add seal-in branches for momentary inputs that should latch.
5. Add the safety, fault, and reset rungs last. Stop buttons (NC, XIC), emergency stops (hardwired AND in the program), fault latches, alarm acknowledge.

This 5-step process scales from a 5-rung program to a 500-rung program. Worked program 10 below uses it from start to finish on a real motor narrative.

Instrumentation and P&ID Symbols

On any process plant — chemicals, food, water treatment, pharma — you’ll be handed a P&ID (Piping and Instrumentation Diagram) before you ever touch the PLC. The P&ID shows every pipe, valve, vessel, pump, and instrument with a standard symbol set defined by ISA-5.1.

An instrument is shown as a circle (or other shape) with a tag inside. The tag has two parts:

• First letter: what’s measured. L=level, P=pressure, T=temperature, F=flow, W=weight.
• Second letter (and beyond): what kind of device. T=transmitter, I=indicator, S=switch, C=controller, V=valve, SH=switch high, SL=switch low.

So LSH-101 is “Level Switch, High, loop 101” — a discrete sensor that closes when level is too high. FT-220 is “Flow Transmitter, loop 220” — an analog 4–20 mA sender. TIC-305 is a Temperature Indicating Controller. Once you can read the tag, you know what to wire to the PLC and how to write the rung.

Putting It Together — From Wiring Diagram to Ladder

You now have all the pieces. The full design loop on a new project looks like this:

1. Read the customer’s narrative and the P&ID. List every input and output device.
2. Build the I/O list — every device gets a PLC address, an electrical specification (NO/NC, voltage, current), and a brief description.
3. Lay out the panel — pick the contactors, motor starters, control transformers, terminal blocks, and PLC chassis.
4. Draw the wiring diagrams — power section, control section (24 V DC), and I/O section (every input and output drawn).
5. Write the ladder logic — section by section, following the narrative. Use the conversion table for any relay-style interlocks pulled from existing equipment.
6. Test in PROG and TEST modes before going to RUN. Force individual inputs from the laptop to verify each rung does what you expect with no real machine motion.

This is the same loop whether you’re writing 30 rungs for a packaging machine or 3 000 rungs for a brewery. The discipline scales. Time now to apply it to ten worked programs.