Keywords: Programmable logic controller; VMEbus; PLC processor module
I. Introduction Programmable logic controllers (PLCs) are being used increasingly in instrumentation and control systems in the process industry, as well as in traditional factory automation environments. They offer industrial ruggedness not found with computers, and a flexibility not found in hardwired control systems. In the early days, the PLC was an expensive electronic relay replacer [1]. Although expensive, it was very beneficial to those who could afford the cost associated with equipment purchase and installation. For this reason, early PLCs were more likely to be found in large manufacturing plants, such as General Motors, General Electric and many others. The design criteria for the first PLC were specified in 1968 by the Hydramatic division of the General Motors Corporation [2]. Since their introduction in 1968, PLCs have steadily gained in popularity in industry. From 1970 to 1974, early innovations in microprocessor technology added greater flexibility and intelligence to the PLCs [2]. Capabilities of operator interfaces (CRT terminals and hand-held programming devices), added new dimensions to the PLC applications. An 8-bit microprocessor-based
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Fig. 1. General PLC system parts.
compiled BASIC. Dudeck [10] used an IBM-PC as a programming device to an industrial PLC manufactured by the Allen-Bradley Company. He illustrated a controlled diffusion pumped vacuum evacuation sequence as a ladder diagram program example for this PC-controlled PLC. Muth [11] showed how an IBM-PC can be used to emulate a PLC, and as an off-line device for control program development and debugging. He also indicated the suitability of the Turbo Pascal as a programming language for the control program as well as for simulation software. Recently, research has been directed towards translating ladder diagram language to other forms of PLC languages. Welch [12] described an algorithm for translating unrestricted relay ladder logic into Boolean form. Krogh and Falcione [13] introduced an algorithm for converting relay ladder logic into a sequential function chart (SFC). Further development in the user interface PC-station with literal languages has been carried out recently. Asmaa [14] designed a literal language compiler (called LitSTAT) for IBM-PC machines which depends on the standard literal statements of many PLC manufacturers. The compiler was written using standard C-language. The PLC is composed primarily of three parts as illustrated in Fig. 1. These parts are: the PLC processor module (PLCPM), the input/output (I/O) modules, and the programming device. The PLCPM and the I/O modules require a housing or a framework to hold and support them. This metal assembly is called the 'rack' or 'crate', which provides a means of supplying power and signals to the PLCPM and I/O modules.
Abstract This paper presents an overall view of the architecture and design of a programmable logic controller (PLC). The main objectives of the work are to design, develop, and implement a versatile PLC processor module (PLCPM) based on an industrial open bus architecture called VMEbus (IEEE 1014 Versa Module Euro-standard). The controller is inserted inside the VME crate and controls the industrial process via input and output modules that reside in the crate. The PLCPM is designed to be an intelligent module through the use of a Motorola MC68000 CPU. A method of distributed arbitration protocol, based on an algorithmic state-machine design approach, is added to the design of this module. This facility makes PLCPM suitable to work inside a VMEbus environment. The PLCPM therefore becomes adequate for use in multiprocessing PLC systems. The controller uses a host personal computer (IBM-PC) as a versatile and indispensable system component for process development, monitoring, control and supervision. Software and firmware programs are developed and written for both host-PC and PLCPM using standard C-language and 68000 assembly language, respectively. This results in a sequential control algorithm for the PLCPM and windowing user interface for the host-PC. Ladder diagram programming language is supported by this user interface. © 1998 Elsevier Science B.V.
Taha F. A1-Khudairy*, Bakir A.R. A1-Hashemy, Munqith A.J. A1-Baker