The programmable logic controller is the workhorse of industrial control systems. It uses digital and analog I/O modules to interface to sensors, actuators and other equipment. These I/O modules must meet stringent electrical specifications and designers face challenges in conditioning and converting these signals. In addition, increasing levels of embedded processing and connectivity can be seen on these modules.
PLCs can be segmented by I/O count and scan rates. They can also be classified as brick type versus modular PLCs. A small or brick PLC has a fixed number of connections for inputs and outputs. 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 is customized to fit the application.
A modular PLC consists of the central processing unit (CPU), a power supply and various I/O modules. The CPU performs the main controller functions like scanning data and running control sequences. The power supply unit converts line power to 24 volts. The I/O modules are used to measure data from sensors and to control various actuators. Together, the input and output modules along with the central controller form a control loop.
I/O modules can be classified as analog input (AI), analog output (AO), digital input (DI) and digital output (DO). They are usually categorized into 2, 4, 8 and 16channel modules. I/O modules need to meet stringent electrical specifications such as the IEC 610004 family of electromagnetic compatibility (EMC) tests for surge, ESD (electrostatic discharge) and EFT (electrically fast transients). They also need short circuit and over load protection.
All these modules include isolation between the control side and the field side via DC/DC converters and isolation on the digital communication side between the field devices and the digital backplane. Communication over the digital backplane is done by a variety of interfaces (RS485, CAN, Ethernet, LVDS, etc.) In distributed control systems or between equipment, industrial field buses (PROFIBUS
, DeviceNet
, CANopen, Modbus, EtherCAT, EtherNet I/P, PROFINET, SERCOS, POWERLINK, etc.) are used for communication.
To learn more about Programmable Logic Controllers, I/O modules, as well as the chipsets and reference designs for these products, please download the attached document by Texas Instruments called "Programmable Logic Control Overview."
| Texas Instruments Programmable Logic control Overview.pdf |
