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Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O.
Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 0 - 10 V input or 4-20 mA would be converted into an integer value of 0 - 32767.
REFERENCES
Pavlovic, R. Pinger and M. Kollmann, “Automated For-mal Verification of PLC Programs Written in IL,” Con-ference on Automated Deduction (CADE), Bremen, July 2007, pp. 152-163.
M. B. Younis and G. Frey, “Formalization of Existing PLC Programs: A Survey,” Proceedings of CESA 2003, Lille, 2003.
N. Bauer, S. Engell, S. Lohmann, M. Remelhe and O. Stursberg, “Verification of PLC Program Given as Se-quential Function Charts,” Lecture Notes in Computer Science, Vol. 3147, 2004, pp. 517-540.
S. R. Koo, P. H. Seong and S. D. Chaa, “Software Design Specification and Analysis Technique for the Safety Critical Software based on Programmable Logic Control-ler (PLC),” Proceedings of the Eighth IEEE International Symposium on High Assurance Systems Engineering (HASE’04), Florida, March 2004, pp. 283-284.
A. Mader and H. Wupper, “Timed Automaton Models for Simple Programmable Logic Controllers,” Proceedings of the 11th Euromicro Conference on Real-Time Systems 1999, York, June 1999, pp.106-113.
E. Brinksma1, A. Mader and A. Fehnker, “Verification and Optimization of a PLC Control Schedule,” Interna-tional Journal on Software Tools for Technology Transfer (STTT), Vol. 4, No. 1, October 2002, pp. 21-33.
S. Lamp′eri`ere and J. J. Lesage, “Formal Verification of the Sequential Part of PLC Programs,” 5th Workshop on Discrete Event Systems (WODES 2000), Ghent, August 2000, pp. 247-254.
S. Kowalewski, S. Engell, J. Preußg and O. Stursberg, “Verification of Logic Controllers for Continuous Plants Using Timed Condition/Event-System Models,” Auto-matica: Special Issue on Hybrid Systems, Vol. 35, No. 3, March 1999, pp. 505-518.
可编程逻辑控制器
可编程逻辑控制器（PLC）或可编程序控制器是用于机电过程自动化的数字计算机，例如控制机械厂生产线、游乐设施或照明装置。可编程控制器在许多工业和机器中使用。与通用的计算机不同的是，PLC是专为多个输入和输出管理，扩展温度范围、不受电磁噪音影响、抗震动和冲击所设计。控制器的操作程序通常存储在电池供电或非易失性的内存中。PLC是实时的系统，因为系统产生的输出结果必须在有限的时间内回馈到输入，否则会导致错误操作。
1.历史
PLC发明是针对于美国汽车制造行业的需要。可编程逻辑控制器最初通过了在软件版本更换硬连线的控制板生产模式更改时的汽车工业。
在PLC之前，控制、程序化和安全联锁逻辑制造汽车是使用上百或上千的继电器、凸轮计时器、鼓定序仪和专用的闭环控制器来完成的。在每年更新模型等设施转变过程是非常耗时并且成本高昂的，这是因为电工需要单独地再接电线给每个中转。
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