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Ladder diagram
Programmable controller language that uses contacts and coils to define a control sequence.
LAN (Local Area Network)
A data communication system which is typically confined to a few kilometers radius, and supports transfer rates up to about 100 Mbps.
Latch
An instruction used in ladder diagram programming to represent an element that retains its state during controlled toggle and power outage.
Latency
The delay time between the end of one communication and the start of another. During this time, the processes associated with the communication are hung up and cannot continue. Software engineers work to reduce latency to a minimum.
Leakage current
Small amount of current that flows through load-powered sensors. The small current is necessary for the operation of the sensor. The small amount of current flow is normally not sensed by the PLC input. If the leakage is too great a bleeder resistor must be used to avoid false inputs at the PLC.
LED (light-emitting diode)
A solid-state semiconductor that emits red, green, or yellow light or invisible infrared radiation.
Light-on sensor
This refers to a photo sensor's output. If the output is on when an object is sensed, the sensor is a light-on sensor.
Line driver
A line driver is a differential output driver intended for use with a differential receiver. These are usually used where long lines and high frequency are required and noise may be a problem.
Line-powered sensor
Normally, three-wire sensors, although four-wire sensors also exist. The line-powered sensor is powered from the power supply. A separate wire (the third) is used for the output line.
Load-powered sensor
A load-powered sensor has two wires. A small leakage current flows through the sensor even when the output is off. The current is required to operate the sensor electronics.
Load resistor
A resistor connected in parallel with a high-impedance load to enable the output circuit to output enough current to ensure proper operation.
Lockout device
A device that utilizes a positive means such as a lock, either key or combination type, to hold an energy isolating device in the safe position and prevent the energizing of a machine or equipment.
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