ISA Certified Control Systems Technician (CCST) program

CCST

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

An “Inclusive OR” function:

A. can never be used with a NOT function.
B. is not applicable to discrete control applications.
C. produces a 1 output if either, or both inputs are 1.
D. produces a 1 output if neither inputs are 1.

CCST Answer

The correct answer is C: produces a 1 output if either, or both inputs are 1.

The resistance measurement displayed by a multimeter is the total resistance through all possible paths between the test lead probes. Measuring resistance across a component that is part of a circuit often requires removal of the component from the circuit.

The “truth table” for an Inclusive OR is (0 = false, 1 = true):

X

Y

X  |  Y

0

0

0

1

0

1

0

1

1

1

1

1

Inclusive OR logic is important for several control system applications.  For example, in a safety system where two pressure switches are used in parallel to detect a high-pressure situation in a 1oo2 arrangement, if the Inclusive OR between the two pressure switch signals is true, then a shutdown situation is detected.

By comparison, an Exclusive OR is "true" whenever either X or Y is true, but not both.

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