Which of the following is not a component of a ladder diagram?

• A. Valve
• B. Contact
• C. Coil
• D. Relay

Answer: (A)

Ladder diagrams model control logic using two fundamental building blocks: contacts and coils. Contacts represent conditions that must be met (like switches or relay contacts) and coils represent outputs that get energized to perform an action. A relay is a device used within the logic to switch other contacts and drive outputs. A valve, while it can be controlled by the ladder logic through an output, is a field/process device rather than a basic ladder diagram element. In ladder diagrams you don’t draw valves as standard components; you depict the control signal (via a coil or contact) that would energize the valve. So the item that isn’t a component of a ladder diagram is the valve.