אוטומציה ובקרה – PLC Topics – Part 1
PLC – Scan Time
A PLC program is generally executed repeatedly as long as the controlled system is running. The status of physical input points is copied to an area of memory accessible to the processor, sometimes called the “I/O Image Table”. The program is then run from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs. This scan time may be a few milliseconds for a small program or on a fast PLC.
PLC – System Scale
A small PLC has a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O.
Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules are customized for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.
PLC- User interface
PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. Human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interface (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring SW installed on a computer, with the PLC connected via a communication interface.
PLC- Communications
PLCs have built-in communications ports and corresponding communications protocols, such as RS-232, EIA-485, Ethernet, Modbus, BACnet or DF1.
Most PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser.
PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link.
Here are some examples of the PLCs used by smartlogic: 6XV1830-0EH10, 6ES7131-4BF00-0AA0,6ES7193-4CA40-0AA0,6ES7134-4GD00-0AB0,6ES7193-4CA40-0AA0, 6ES7138-4CA01-0AA0,6ES7193-4CC20-0AA0, 6ES7590-1AB60-0AA0, 6ES7511-1AK00-0AB0, 6ES7954-8LP01-0AA0,6ES7155-6AU00-0BN0
