1 July 2005

ISA Certified Control Systems Technician (CCST) Program

This question is from the Level III study guide, Domain 2, Loop checking.

Level III represents a professional who has a 13-year total of education, training, or experience.

CCST question

Programmable Logic Controllers:

A. Use sequential logic control.

B. Perform PID control with the same capability as DCS.

C. Have limitations in flexibility of control strategies.

D. Are programmed using flow charts and P & IDs.

CCST answer

We use ladder logic to program programmable logic controllers (PLCs), where a series of complex logic checks are required before something is turned on. Ladder logic is useful for simple but critical control systems or for reworking old hardwired relay circuits. As PLCs became more sophisticated, it has also been used in very complex automation systems.

Ladder logic (sequential logic) is a rule-based language rather than a procedural language. A "rung" in the ladder represents a rule. When implemented with relays and other electromechanical devices, the various rules "execute" simultaneously and immediately.

The rules are typically executed sequentially by software in a loop. The best answer is A.

Nicholas Sheble (nsheble@isa.org) explains the answers to CCST questions. For information about the CCST program, go to www.isa.org/ccst.