July/August 2011

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

In a PLC program, the continuous sequence of steps or "operating cycle" performed by a PLC processor is known as:

A.        Updating

B.         Polling

C.        Scanning

D.        Rectifying

CCST answer

The correct answer is C, scanning. The time it takes for the PLC to complete its scanning of all program steps in one "operating cycle" is called the "scan time."

Answer A is not the correct choice. Updating is a general term used for making updates to a software element value, program, or file.

Answer B, polling, is a method for sequentially observing each channel of a network or device in order to determine if it is ready to receive data or for requesting computer action..

Answer D, rectifying, refers to the conversion of alternating current to a relatively steady or pulsating direct current. There is most likely a rectifier circuit in the power supply that powers the PLC or PLC I/O.