Ladder Diagram Programming — Fundamentals

What Is a Ladder Diagram?

A ladder diagram (LD) is one of the five IEC 61131-3 PLC programming languages. Its name comes from its visual shape: two vertical rails (like the sides of a ladder) connected by horizontal rungs. Each rung is a logic circuit. The PLC scans all rungs from top to bottom, left to right, in a continuous loop called the scan cycle. On each scan the processor reads all physical inputs, solves the logic, and updates all physical outputs.

Ladder diagrams were designed to look like the relay-logic drawings electricians already knew, making them the most common starting point for learning PLC programming in manufacturing and mechatronics.

Core Instructions

XIC — Examine If Closed. Passes power (logic continuity) when the tag it references is TRUE (bit = 1). On a motor starter, the Start pushbutton input tag uses an XIC so that logic flows only when someone presses the button.

XIO — Examine If Open. Passes power when the tag it references is FALSE (bit = 0). The Stop pushbutton is wired normally-closed (NC) in hardware, so its input tag is normally TRUE; in the program a matching XIC passes power at rest. Alternatively, a normally-open Stop button can be wired and its tag referenced with XIO — power passes only while the button is NOT pressed. Either wiring scheme is valid; know which one your trainer uses before programming.

OTE — Output Energize. The standard output coil. When the rung goes TRUE the output bit is set to 1; when the rung goes FALSE the bit resets to 0. Suitable for momentary control.

OTL — Output Latch. A retentive set instruction. Once the rung goes TRUE, the output bit stays at 1 even if the rung later goes FALSE. Only an OTU on a separate rung can reset it.

OTU — Output Unlatch. The retentive reset instruction paired with OTL. When its rung goes TRUE the bit resets to 0 and stays there until OTL fires again.

The Seal-In (Memory) Circuit

A motor starter needs to keep running after the operator releases the Start button. The classic three-rung approach uses a seal-in (holding) contact:

• Rung 1 (Start logic): XIC(Stop_NC) in series with a parallel branch. The parallel branch contains XIC(Start) in parallel with XIC(Motor_Run). The output is OTE(Motor_Run).
• The Motor_Run coil drives a contactor coil through the output module.
• When Start is pressed, Motor_Run goes TRUE. The XIC(Motor_Run) contact in parallel with Start now also passes power, so releasing Start does not drop the rung — this is the seal-in contact.
• When Stop is pressed (opening the NC contact or activating the XIO logic), the rung goes FALSE, Motor_Run drops, and the seal-in contact opens — the motor stops.

A second rung (overload) and a third rung (indicator light) can be added for a complete three-rung program, but the seal-in logic lives in that first rung.

The Latched Output Circuit

For a pilot light that must stay on independently of any button, use OTL/OTU:

• Rung A: XIC(Latch_PB) → OTL(Pilot_Light)
• Rung B: XIC(Unlatch_PB) → OTU(Pilot_Light)

Pressing the Latch pushbutton fires OTL and the light stays on. Pressing the Unlatch pushbutton fires OTU and the light goes off. This is useful for alarms, mode indicators, and any condition that must be acknowledged before resetting.

Online Monitoring and Debugging

After downloading your program, go online with the PLC. The software shows green (highlighted) contacts when their bits are TRUE and gray contacts when FALSE. This lets you trace logic flow rung by rung without a voltmeter — but a DMM is still your primary tool for confirming actual wire voltage at terminals.

When debugging, work systematically:

1. Check the online view first — which contact is not passing when you expect it to?

2. If the input contact shows no highlight even when the field device is actuated, suspect wiring or the wrong tag address.

3. Confirm input voltage with a DMM at the input terminal (check that voltage is within spec for the input module — typically 24 V DC on training hardware).

4. If the output coil shows TRUE but the device does not energize, suspect output wiring or a blown fuse on the output module.

Safety: Lock Out Before You Touch Hardware

Before connecting or disconnecting any wiring on the PLC trainer, you must follow OSHA 29 CFR 1910.147 (Control of Hazardous Energy). Apply your personal lock to the energy-isolation device, verify stored energy is released, and verify absence of voltage with a meter before touching terminals. NFPA 70E requires you to verify the absence of voltage with a properly rated and tested meter — never assume a circuit is dead because the software shows outputs as OFF. The PLC can re-energize an output the instant a scan completes.

Inspect all terminal connections for bare conductors that could create a shock or arc hazard. Wear safety glasses whenever the trainer is energized. Machine guarding requirements (OSHA 1910.212) apply once actual motor loads are running — keep hands clear of any rotating parts during tests.

Download and Verify Procedure

1. Verify your tags match the physical I/O map on the trainer label.

2. Save your project file before going online.

3. Put the PLC in Program mode, download, then switch to Run mode.

4. Test each pushbutton in sequence; confirm the online view matches your expectation.

5. Log the results — observed behavior, any faults found, corrective action taken — in your lab log and have the instructor initial it.