Rung
Quick answer
A ladder-logic rung is one network of conditions and actions evaluated as part of a PLC routine. Power-flow notation generally reads left to right, while rung and routine execution order follows the controller’s scheduled task and platform rules.
Key Takeaways
- A ladder-logic rung is one network of conditions and actions evaluated as part of a PLC routine. Power-flow notation gen...
- Beginner-level topic in Programming Languages
- Commonly used in: Motor command and permissive logic, Alarm and status evaluation
Detailed Definition
A rung contains contacts, comparisons, branches and instructions arranged to express one control decision. The left side represents the conditions; output or action instructions usually appear toward the right. A clear rung has one purpose, meaningful tag names and a comment that explains intent rather than merely restating symbols.
Do not assume a rung is isolated. A later rung can write the same tag again, a called routine may change its source conditions and task scheduling can affect when it runs. Review the complete write path and execution context when troubleshooting.
Evidence and review status
The page uses general ladder-logic conventions. Branch evaluation, output update and multiple-write diagnostics vary by PLC platform.
Technical review date:
Critical behavior to understand
- Conditions generally evaluate left to right, but task, routine and rung order determine when the network executes.
- Parallel branches implement logical alternatives; series contacts implement combined conditions.
- Multiple writes to one tag make the final state dependent on execution order.
- A latched command needs a defined reset path and startup-state decision.
Verification checklist
- 1State the rung’s single intended outcome in plain language.
- 2Search all routines and tasks for writes to the same destination.
- 3Test first scan, false-to-true transitions and loss of every permissive.
- 4Confirm the displayed online state matches the task currently executing.
Worked example
Verify a non-safety motor seal-in rung
StartPB or MotorCommand is in a parallel branch, in series with StopHealthy and ProcessPermissive, driving MotorCommand.
- 1Begin with StopHealthy and ProcessPermissive true and MotorCommand false.
- 2Pulse StartPB true for one scan and then release it.
- 3Set ProcessPermissive false.
- 4Restore the permissive and confirm the restart policy matches the design.
Expected result: MotorCommand seals in after StartPB, drops when the permissive is lost and only restarts according to the documented reset or restart policy.
Common Questions
What is Rung?
A ladder-logic rung is one network of conditions and actions evaluated as part of a PLC routine. Power-flow notation generally reads left to right, while rung and routine execution order follows the controller’s scheduled task and platform rules.
When should I use Rung?
Rung is particularly useful in scenarios such as Motor command and permissive logic and Alarm and status evaluation. Consider implementing it when you need reliable, efficient solutions for these types of applications.
What should I verify before using Rung?
State the rung’s single intended outcome in plain language. Search all routines and tasks for writes to the same destination. Test first scan, false-to-true transitions and loss of every permissive. Confirm the displayed online state matches the task currently executing.
What are related concepts I should learn?
To fully understand Rung, you should also familiarize yourself with Ladder Logic, Structured Text, and Function Block Diagram. These concepts work together in industrial automation systems.
Continue Learning
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Quick Info
- Category
- Programming Languages
- Difficulty
- Beginner
- Tier
- Essential
About Programming Languages
IEC 61131-3 standard programming languages and syntax