ISA Certified Control Systems Technician (CCST) program


Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

The function of an A/D converter in a PLC analog input card is to:

A. convert the input power from ac to dc.
B. convert the output power from dc to ac.
C. convert the input analog signal to digital format.
D. convert the output digital signal to analog format.

CCST Answer

The correct answer is C: "convert the input analog signal to digital format".

An analog-to-digital converter (A/D or ADC) accepts an analog input, either a voltage or a current, and converts it to a digital value that can be read by a microprocessor, such as a PLC.

In process control, the input to the A/D is typically a 4-20ma analog signal. By using a 250 ohm resistor, the signal is converted to 1-5 VDC and is connected to the input to the A/D circuit. The input signal is then compared to a reference voltage and “digitized”, meaning it is converted to a binary, digital value. This digital value is typically is a “word” comprised of 8, 16, or 32 bits.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Press