In many industrial control systems, it is required to activate an output function whenever any one of several inputs is activated. For example, in a chemical process it may be desired that an alarm be activated whenever the process temperature exceeds a maximum value or whenever the pressure goes above a certain limit. Figure 3-4 is a block diagram of this situation. The temperature transducer circuit produces an output voltage proportional to the process temperature. This voltage, V_T, is compared with a temperature reference voltage, V_TR, in a voltage comparator circuit. The comparator output, T_H, is normally a low voltage (logic 0), but it switches to a high voltage (logic 1) when V_T exceeds V_TR, indicating that the process temperature is too high. A similar arrangement is used for the pressure measurement, so that its associated comparator output, P_H, goes from LOW to HIGH when the pressure is too high. What is the purpose of the OR gate?

Figure 3-4

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Temperature transducer Pressure transducer i Chemical process VTR VPR Comparator Comparator TH PH Alarm