Wednesday, March 2, 2011

Software Based On PLC and PC Philosophy

Software Based On PLC Philosophy
The vendors of traditional PLC software start with a reliable and easy-to-use scanning architecture and work to add new functionality. PLC software follows a general model of running control code, scanning inputs, updating outputs, and performing housekeeping functions. A control engineer is concerned only with the control code design because the input cycles, output cycles, and housekeeping cycles are all hidden. This strict control architecture makes it faster and easier to create control systems with much of the work done by the vendor. The systems rigidity also eliminates the need for the control engineer to completely understand the low-level operation of the PLC to create reliable programs. Most PLC vendors create PAC software by adding into the existing scanner architecture new functionality such as motion control, Ethernet communication, and advanced algorithms. However, typically they maintain the familiar look and feel of PLC programming and the inherent strengths in control and logic. The result is PAC software usually designed to fit specific types of applications such as motion, logic, and PID, but is less flexible for custom applications such as data logging, communication, or custom control algorithms.

Software Based On PC Philosophy
Traditional PC software vendors start with a very flexible programming language, which provides in-depth access to the inner workings of the hardware. This software also incorporates determinism, reliability, and default control architectures. Although engineers can create the scanner structure provided normally to the PLC programmer, they are not inherent to PC-based control software. This makes the PC software well suited and extremely flexible for complex applications that require programming techniques, advanced structures, or system level control but more difficult for simple applications.

The step for these vendors is to provide determinism and reliability, which are often not available in a general-purpose operating system such as Windows. This is accomplished through RTOS (real-time operating systems) such as Phar Lap from VxWorks from or Wind River Ardence (formerly Venturcom).

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