Ladder Logic Contacts, Coils and Power Flow

Direct answer

In ladder logic, a contact reads or tests a Boolean value and a coil writes a Boolean value. A direct contact (--| |--) passes the incoming rung condition when its assigned bit is 1/TRUE. A negated contact (--|/|--) passes it when the assigned bit is 0/FALSE. Contacts in series implement an AND relationship; parallel branches implement an OR relationship. The resulting rung condition reaches a coil or another output instruction on the right.

An ordinary coil normally writes both states: true rung condition writes 1, and false rung condition writes 0. A set/latch coil writes 1 only when enabled; its false rung does not undo the prior set. A reset/unlatch coil writes 0 only when enabled. Exact prescan, postscan, retention and restart behavior is controller-specific.

The contact symbol is a software test, not a physical contact specification. A negated ladder contact does not prove that the field device is physically normally closed, de-energize-to-trip, fail-safe or part of a safety function. Trace the full chain: physical state → electrical input → raw PLC bit → mapped/application bit → contact truth → rung result → coil writer → output mapping → physical feedback.

Read ladder power flow as Boolean evaluation

Power flow is a model, not electrical current

Ladder Diagram (LD) inherited the visual grammar of relay schematics: vertical rails, horizontal rungs, contact-like conditions and coil-like results. IEC 61131-3 defines Ladder Diagram as a graphical programmable-controller language. The left rail supplies a conceptual TRUE condition into a network; the software evaluates elements and connections toward the right.

No electrical current flows through a drawn software contact. Online editors often color a path green or blue to show a true rung condition, but that highlight only reports evaluated program state. It cannot prove input voltage, output current, actuator motion, network health, execution frequency or the absence of another later writer.

Separate bit state, contact truth and rung truth

Three values can be confused:

1. Bit state is the assigned variable's current 0 or 1.
2. Contact truth is the direct or negated test result for that bit.
3. Rung-condition-out is contact truth combined with the incoming path and every series/parallel element before it.

A direct contact assigned to a 1 bit can still sit on a false branch if an earlier series condition is false. A negated contact assigned to 0 is true, yet the final rung may remain false because another required path is missing.

Apply the contact truth table

Direct contact tests for one

A direct contact is true when its assigned Boolean is true. Rockwell calls this XIC, Examine If Closed; other systems may call it normally open, direct contact, examine-on or simply contact. The most portable mental model is “test whether the assigned Boolean equals TRUE.”

Negated contact tests for zero

A negated contact is true when its assigned Boolean is false. Rockwell calls it XIO, Examine If Open; other systems use normally closed, inverted or negated contact. Read it as “test whether this Boolean equals FALSE,” not as a claim about field hardware.

Translate physical devices in two steps

Suppose a pushbutton has a physical normally closed contact and the PLC input bit is 1 while the circuit is healthy. A direct ladder contact on that input is true in the healthy state. If the wire opens or the button is pressed, the input falls to 0 and the direct ladder contact becomes false. The physical NC contact led to a direct software test—not a negated one.

Conversely, a physical normally open sensor can produce a bit that is 0 until actuated. A negated contact on that bit would be true while the sensor is inactive. Physical contact form and ladder symbol are two different layers.

Combine contacts with series and parallel paths

Series contacts implement AND

Two direct contacts A and B in series allow a true path only when A = TRUE and B = TRUE. Adding a negated contact changes only its operand test. For example, direct A in series with negated B means A AND NOT B.

Parallel branches implement OR

Two contacts A and B on parallel branches produce a true combined path when either branch is true. The logic is A OR B. If each branch contains series elements, solve each branch first and then OR the branch results.

Do not simplify a network by visual impression. Write the Boolean expression or enumerate a truth table. Pay particular attention to branches that join after an interlock: a misplaced branch can bypass a condition that was intended to constrain every path.

Understand ordinary, set and reset coils

Ordinary coil writes both true and false

An ordinary IEC coil stores the incoming Boolean result. Rockwell OTE sets or clears its target according to the rung condition. Schneider and CODESYS describe the ordinary coil similarly. Therefore the false state of the rung is an active write of false, not “nothing happens.”

This matters when two ordinary coils target the same bit: the later executed writer can overwrite the earlier result. It also matters when a routine stops executing; a bit may no longer be written in the same way, and platform-specific prescan/postscan or task behavior becomes relevant.

Set and reset coils write one state selectively

A set/latch coil writes TRUE when its rung is true and normally performs no opposite write while its rung is false. A reset/unlatch coil writes FALSE when its rung is true and normally performs no opposite write while its rung is false. They form explicit state memory only when ownership, initialization and reset paths are designed.

“Retentive” in an instruction description does not automatically guarantee power-loss persistence, download behavior or nonvolatile storage. Those are controller, memory, project and configuration properties. Test transitions through program/run mode, power cycle, download, first scan and fault recovery where relevant.

Build and critique a seal-in rung

See how the holding contact works

A conventional start/stop seal-in pattern has a momentary start request in parallel with a contact that reflects the run memory, while a stop/permit condition constrains both paths. On start, the coil becomes true. On the next evaluation, the run contact becomes true and maintains the path after the start request is released. Removing the stop/permit path makes the ordinary coil false, which opens the run contact.

Test the cases the textbook diagram omits

A seal-in rung is ordinary command memory, not a safety function. Emergency stop, guard monitoring and other required risk-reduction functions need the validated safety system, appropriate devices, diagnostics, architecture and lifecycle.

Treat scan order and multiple writers as first-class behavior

Later writes can win within one execution

Many PLC programs execute ordered rungs/routines. If rung 1 writes a bit true and a later rung 2 writes the same bit false, later logic can determine the value observed after those executions. Siemens explicitly warns that when the same coil address appears in multiple program locations, the last calculation can determine the physical output update. Rockwell warns about operands being overwritten.

The exact scheduler can be more complex than a single top-to-bottom scan: tasks can preempt, routines can be conditional, I/O can update asynchronously and external systems can write tags. “Last rung wins” is therefore a useful local warning, not a universal concurrency model.

Find every direct and indirect writer

Use cross-reference and instruction usage tools, then include indirect writers:

One final command owner is easier to test: other routines produce requests, permissives and interlocks; one arbitration routine writes the final application command; one mapping layer writes the physical output.

Use edge contacts and one-shots deliberately

An edge is an event relative to stored history

A rising-edge instruction is true for one execution when a condition changes from false to true according to its stored prior state. A falling-edge instruction responds to true-to-false. Rockwell provides ONS/OSR/OSF variants; IEC systems provide rising/falling transition contacts or function blocks according to implementation.

“One scan” means one execution of that instruction in its task/routine context. If the routine is called conditionally or the task runs at a different interval, the observed pulse and history behavior can differ from a naive continuous-scan assumption.

Troubleshoot from field condition to feedback

Follow the evidence chain left to right

Capture a reproducible ladder incident

Record controller/project identity, mode, task/routine execution, timestamp, raw input and quality, mapped tags, rung conditions, coil target, every writer, forces, output module state and physical feedback. A screenshot without project revision, time and tag values is weak evidence. A trend can reveal changes between screen refreshes.

Use the PLC input and output troubleshooting guide for the physical-to-tag path and the intermittent PLC fault guide for time-correlated evidence.

Review ladder rungs for maintainability

Apply a rung-quality checklist

Long rungs are not automatically bad, but invisible ownership is. Break reusable calculations or equipment behavior into clear units without hiding the decisive final command path. Expose reason bits for permissives and interlocks so operators see why a rung is false.

Diagnostic answer map for search and AI-assisted troubleshooting

Frequently asked questions

What is the difference between a contact and a coil in ladder logic?

A contact reads/tests a Boolean condition and contributes to rung truth. A coil writes a Boolean according to the final incoming rung condition and its coil type. Contacts do not write their assigned bit; coils do.

When is a normally open ladder contact true?

A direct or normally open ladder contact is true when its assigned Boolean is 1/TRUE and the incoming rung path is true. The symbol does not tell you the physical device's contact arrangement.

When is a normally closed ladder contact true?

A negated or normally closed ladder contact is true when its assigned Boolean is 0/FALSE and the incoming path is true. Many vendors use different names, so verify the instruction semantics.

Why use a normally closed physical stop but a direct PLC contact?

If the healthy physical NC loop energizes the PLC input, its input bit is 1 while healthy. A direct ladder contact then tests that healthy 1. Opening the circuit makes the bit and direct contact false. The exact circuit and safety role still require engineered documentation.

Do two contacts in series always mean AND?

They AND their individual Boolean test results along that path. A negated contact contributes the inverse of its assigned bit, so direct A in series with negated B represents A AND NOT B.

Can I use the same output coil more than once?

Many platforms allow it, but later or higher-priority writers can overwrite earlier results and make troubleshooting dependent on execution order. Prefer a single documented final owner and explicit request/arbitration logic.

What is the difference between a seal-in coil and a latch coil?

A seal-in pattern uses an ordinary coil plus its own contact in a parallel holding path; when the constraining path becomes false, the coil writes false. A latch/set instruction makes a one-sided true write and needs a separate reset/unlatch action.

Does a latched PLC bit survive power loss?

Not necessarily. Latch semantics describe what the instruction does when executed or not enabled. Power-loss persistence, downloads, controller mode transitions and nonvolatile behavior depend on the exact controller, memory and configuration.

What does one scan mean for a one-shot?

It means one execution interval of that instruction in its task/routine context. A conditionally called routine or different task period changes the real time and history behavior, so test the exact scheduler.

Why is the output coil true but the machine does not move?

The coil may be an internal command rather than the physical output, a later writer may change it, output mapping/interlocks may block it, or the module/load/process may be faulty. Verify every boundary through physical feedback.

Sources, review scope, and limitations

This guide was reviewed on August 28, 2026 against current public sources. Ladder syntax and execution differ across vendors, controller families, firmware, task schedulers, editors and language profiles. Verify the exact platform and tested project.

• IEC 61131-3:2025 programmable-controller programming languages — IEC
• Studio 5000 Logix Designer v38 bit instructions — Rockwell Automation
• Output Energize (OTE) execution — Rockwell Automation
• Output Unlatch (OTU) execution — Rockwell Automation
• Logix 5000 Controllers Ladder Diagram, 1756-PM008 — Rockwell Automation
• Logix 5000 Controllers IEC 61131-3 Compliance, 1756-PM018 — Rockwell Automation
• Logix 5000 Controllers General Instructions, 1756-RM003 — Rockwell Automation
• Contact and coil behavior — Schneider Electric
• Set and reset coil behavior — Schneider Electric
• IEC ladder programming language elements — Schneider Electric
• CODESYS ladder coil, negated coil and set/reset behavior — CODESYS
• S7-1200 Easy Book ladder contact, coil and duplicate-address explanation — Siemens
• 29 CFR 1910.147, control of hazardous energy — OSHA
• 29 CFR 1910.333, electrical work practices — OSHA

The figures are conceptual editorial teaching diagrams, not vendor screenshots, compilable project code, electrical schematics, safety logic or field procedures. Symbols and labels illustrate Boolean relationships only. This page does not authorize online edits, downloads, forces, bypasses, output commands, rewiring, energized measurement or machine testing. Only qualified, authorized personnel following the site risk assessment, change control, hazardous-energy and electrical safe-work procedures, validated safety lifecycle, engineered drawings, exact manufacturer documentation and approved test plan should modify or test an installed control system.