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Ladder logic is widely used to program PLCs, where sequential control of a process or manufacturing operation is required. Ladder logic is useful for simple but critical control systems or for reworking old hardwired relay circuits. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems.
In the water level controller sample program shown in the page high level and low level float switches are open (input low) when water is above them and vice versa. That means for the fill valve to be latched open both inputs should be high. But this sample program shows both input should be low for the valve to be latched. Can anyone check?
It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and occur only in a fault condition or during a logic-level transition. However, few logic circuits can detect such a condition, and most devices will interpret ...
Example Ladder Logic Diagram. The schematic diagrams for relay logic circuits are often called line diagrams, because the inputs and outputs are essentially drawn in a series of lines. A relay logic circuit is an electrical network consisting of lines, or rungs, in which each line or rung must have continuity to enable the output device. A ...
TTL logic inputs that are left unconnected inherently float high, and require a much lower valued pull-down resistor to force the input low. A standard TTL input at logic "1" is normally operated assuming a source current of 40 μA, and a voltage level above 2.4 V, allowing a pull-up resistor of no more than 50 kohms; whereas the TTL input at ...
High-level and low-level, as technical terms, are used to classify, describe and point to specific goals of a systematic operation; and are applied in a wide range of contexts, such as, for instance, in domains as widely varied as computer science and business administration.
An equivalent high-level circuit uses logic gates, each represented by a different shape (standardized by IEEE/ANSI 91–1984). [45] A low-level representation uses an equivalent circuit of electronic switches (usually transistors). Most digital systems divide into combinational and sequential systems. The output of a combinational system ...
If all inputs are high, each buffer will be in a high-impedance state and the pull-up resistor will pull the output high. But if any input is low, the output will be pulled low by the buffer for that input. This corresponds to wired AND in active-high logic, or to wired OR in active-low logic, and allows multiple inputs to share the same output ...