The physical behavior of a switch changes the logic you have to write behind it. A momentary contact is active only while it is physically held and springs back the instant you let go, while a maintained contact stays in whatever position you left it until something moves it again. That single mechanical difference decides whether your rung needs memory to hold an output on, or whether the switch itself already holds the state for you. Confusing the two is a common source of controls that either will not stay on or refuse to turn off.
Maintained vs momentary contact in one line: A momentary contact is active only while it is physically actuated and returns to its rest position as soon as it is released, like a spring-return pushbutton. A maintained contact stays in its selected position until it is deliberately moved again, like a selector switch or a twist-to-release e-stop. Because a momentary input disappears on the next scan, logic driven by one usually needs latch or seal-in memory to keep an output on, while a maintained input holds the state on its own.
A momentary contact is the ordinary pushbutton feel: you press it, the contact makes, you release it, and a spring returns it, breaking the contact. To the PLC the input is true for only as long as a finger is on the button, which might be a fraction of a second and only a handful of scans. That is fine when the button's job is to command a brief event, but it is a problem when the button is meant to start something that should keep running after the operator walks away. The input simply will not be there on the next scan.
A maintained contact holds its state mechanically. A two-position selector left in the run position keeps its contact made until someone physically turns it back, so the PLC sees a steady, continuous input with no logic tricks required. Because the switch itself remembers the operator's choice, the rung can drive the output directly and it will stay on as long as the switch stays put. Maintained devices include selector switches, toggle switches, key switches, and mushroom-head emergency stops that latch in when struck and hold until twisted to release.
The consequence for programming is direct. A maintained input can often feed a normal coil and behave correctly, because the input persists on its own. A momentary input almost always needs some form of memory around it, either a seal-in path where the output holds itself in through one of its own contacts, or an explicit latch instruction, so that the brief press is captured and held. Deciding which kind of switch is on the other end of a terminal is therefore the first thing you need to know before writing the rung, not an afterthought.
The textbook start/stop station combines both behaviors deliberately. The start button is momentary and spring-return, so it cannot leave the machine commanded on just because someone leaned on it, and the logic captures the press into a held run state. The stop button is also momentary but wired to break the hold, so a quick press drops the run state and the machine stops. The operator gets clean, positive control from two brief presses, and the memory to hold the run state lives in the logic rather than in the buttons.
An emergency stop is intentionally different. A proper e-stop is a maintained, latching device: when it is struck, the mushroom head locks in the actuated position and stays there, holding the machine in its stopped state, and it cannot be cleared by simply releasing pressure. It takes a deliberate action, usually a twist or pull, to unlatch it, and many designs then require a separate reset before equipment can restart. This is by design, so that a hazard cannot quietly reset itself and so that clearing the stop is always a conscious human act at the device.
That maintained-by-design behavior is a safety principle, not a convenience. If an e-stop were momentary, the machine could become free to restart the instant the operator's hand came off the button, which is the opposite of what an emergency stop is for. Because it latches mechanically, the stopped condition persists on its own regardless of what the logic is doing, and the person who hit it, or someone who has verified the area is safe, must go and deliberately release it. Treating an e-stop as anything other than a maintained, latched input is a fundamental mistake.
The classic bug appears when a programmer assumes a momentary input is maintained. They wire a spring-return start button straight to a coil, or map a momentary remote command from a SCADA screen directly to a run bit, expecting it to stay on. It works for the instant the button is held or the command is sent, then the input goes false on the very next scan and the output drops out. The equipment starts and immediately stops, or a remote command appears to do nothing, and the cause is not a fault but a missing piece of hold logic.
The reverse mistake also happens: treating a maintained switch as if it were momentary and wrapping unnecessary latch logic around it. That can leave an output latched on after the operator has already returned the selector to off, because the code is holding a state the switch is no longer commanding. The output and the physical switch position disagree, which is confusing to diagnose and can mask the true intent of the operator. The fix in both directions is the same: identify the real behavior of the device and match the logic to it exactly.
This matters especially for remote and cloud-monitored operations, where commands often arrive as momentary pulses over a link. A command sent from a dashboard is by nature a brief message, not a held contact, so the receiving logic must latch that pulse into a maintained state on the controller and reflect the true held state back to the operator's screen. When a remote command seems to have no effect, or a status indicator flickers and reverts, the underlying issue is very often a momentary signal that was never captured into a maintained bit. Getting maintained versus momentary right is what makes remote control feel reliable.
Yes, if the button is meant to start something that should keep running after release. A momentary contact is only true while it is held and goes false on the next scan, so the logic needs a seal-in path or a latch instruction to capture the press and hold the output on. Without that memory, the equipment starts and immediately stops when the button is let go.
An e-stop is maintained and latching so the stopped condition cannot reset itself. When struck, the mushroom head locks in the actuated position and holds the machine stopped until a person deliberately twists or pulls to release it, often followed by a separate reset. If it were momentary, the machine could become free to restart the instant pressure came off the button, which defeats the purpose of an emergency stop.
Actuate it and see what happens when you let go. A momentary device springs back to its rest position on release, while a maintained device stays where you left it until moved again. Selector switches, key switches, and latching mushroom e-stops are maintained, while ordinary spring-return pushbuttons are momentary. The device datasheet or catalog description also states the action directly.
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