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The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a fieldbus or computer network that plugs into the PLC. The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control , distributed control systems , and networking .
Part 3 of IEC 61131 deals with basic software architecture and programming languages of the control program within PLC. It defines three graphical and two textual programming language standards: Ladder diagram (LD), graphical; Function block diagram (FBD), graphical; Structured text (ST), textual
The AS-Interface is a single-master system, this means a master device exchanges the input and output data with all configured devices. The transmission medium is an unshielded two-wire yellow flat cable. The cable is used for signal transmission and at the same time for power supply (30 V).
SIMATIC devices have input and output modules to connect with controlled machines. The programs on the SIMATIC devices respond in real time to inputs from sensors on the controlled machines, and send output signals to actuators on the machines that direct their subsequent operation.
A simplified PAL device. The programmable elements (shown as a fuse) connect both the true and complemented inputs to the AND gates. These AND gates, also known as product terms, are ORed together to form a sum-of-products logic array. A programmable logic device (PLD) is an electronic component used to build reconfigurable digital circuits.
The "coil" (output of a rung) may represent a physical output which operates some device connected to the programmable controller, or may represent an internal storage bit for use elsewhere in the program. A way to recall these is to imagine the checkers (contacts) as a push button input, and the actuators (coils) as a light bulb output.
A feedback control loop is directly controlled by the RTU or PLC, but the SCADA software monitors the overall performance of the loop. For example, a PLC may control the flow of cooling water through part of an industrial process to a set point level, but the SCADA system software will allow operators to change the set points for the flow.
The Sercos (serial real-time communication system) interface is a globally standardized open digital interface for the communication between industrial controls, motion devices (drives) and input output devices (I/O). Sercos I and II are standardized in IEC 61491 and EN 61491.
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