Bookmark and Share January/February 2013 Certification Review
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 main
purpose of an optical isolation circuit on a digital input card
in a PLC I/O system is:  A.     to provide a common reference
point for DC signals.  B.     to isolate the low-voltage
circuitry on the digital input card from the field device
voltage.  C.     to block light from the surroundings in order
to prevent the digital input card from overheating.  D.     to
isolate the positive and negative terminals on the digital input
card.  CCST answer  The correct answer is B, “to isolate the
low-voltage circuitry on the digital input card from the field
device voltage.” An optical isolation circuit is used to provide
a barrier between the field wiring and as