Question 66 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is A **Explanation for Option A (Correct Answer):** The block labeled "ANALOG D/A" (Digital-to-Analog Converter) within the output system of a Direct Digital Control (DDC) configuration serves a specific purpose. The CPU (Central Processing Unit) operates using digital logic and generates digital control signals. However, many final control elements (like valves or dampers) used in industrial processes are analog actuators, requiring an analog input signal (usually 4-20 mA or 0-10 V) to operate. Therefore, the "ANALOG D/A" block receives the digital output signal from the CPU and converts this digital information into a proportional analog voltage or current signal suitable for driving the analog actuators. **Explanation of Incorrect Options:** * **B) It receives analog outputs from the CPU and conditions these to analog signals for transmission to the analog actuators.** * This is incorrect because the CPU inherently produces digital, not analog, outputs. Conditioning analog outputs implies amplification or filtering, but the primary function here is conversion from digital to analog. * **C) It receives digital outputs from the CPU and conditions these to digital signals for transmission to the digital actuators.** * This is incorrect because the block is specifically labeled "ANALOG D/A," indicating an analog output, not a digital output for digital actuators. If the output were digital, a simple digital interface or conditioning unit would be used, not a D/A converter. * **D) It receives analog outputs from the CPU and converts these to digital signals for transmission to the digital actuators.** * This is incorrect for two reasons: the CPU output is digital, not analog, and the block's function is D/A (Digital-to-Analog), not A/D (Analog-to-Digital).