ISA Certified Automation Professional (CAP) Questions


Using Modbus, what is the MAXIMUM level of precision available?
A. 8-bits
B. 16-bits
C. 24-bits
D. 32-bits


The correct answer is B, 16-bits.

Common Modbus uses two 8-bit bytes for each data register.
2 x 8 bits = 16 bits

To achieve more precision special programming or a specialized version of Modbus is necessary.

Eight bits is less precise than 16-bits; 24-bits and 32-bits require uncommon means to achieve.

Reference: Bella Liptak, Instrument Engineers Handbook, Volume 3 (3rd Edition), Process Software and Digital Networks; ISA, 2002

Modbus transmits over serial lines between devices. The simplest setup would be a single serial cable connecting the serial ports on two devices, a Master and a Slave.

The data transmits as series of ones and zeroes (bits). Each bit is a voltage. Zeroes are positive voltages, and ones are negative voltages. The bits transmit very quickly. A typical transmission speed is 9600 baud (bits per second).

Author - David

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  1. The method by which the tasks and hazards associated with a machine or process are analyzed is known as:

A. Risk assessment.
B. Machine assessment.
C. Risk reduction.
D. Risk abatement.

  1. To test controller tuning or prototype new control strategies offline, the model should be a(an):

A. Tie-back (loopback) simulation.
B. Artificial neural network.
C. Dynamic process simulation.
D. Steady state process simulation.

  1. The temperature measurement with the BEST repeatability and resolution is the:

A. Thermocouple.
B. Resistance temperature detector (RTD).
C. Dial thermometer.
D. Capillary system.

  1. Which of the following is NOT a variable speed drive setup parameter?

A. Acceleration rate.
B. Motor winding type.
C. Output frequency.
D. Maximum speed.

  1. A complete test plan for system integration testing MUST include:

A. Comments for the application programmer.
B. Multiple test cases for each mode of operation.
C. At least five test cases for each test.
D. Expected results for each test case.

  1. Frequency of maintenance should be determined by:

A. Failure rates of components.
B. Availability of personnel and parts.
C. Management targets for efficiency and productivity.
D. Effectiveness of maintenance personnel.


1 = A
2 = C
3 = B
4 = B
5 = C
6 = A

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