ISA Certified Automation Professional (CAP) program

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Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

Which type of communications error protection and correction uses a complex polynomial that results in a frame check sequence being added to a message?
A. checksum
B. cyclic redundancy check
C. error correcting code
D. parity error correction

CAP Answer

The correct answer is B, cyclic redundancy check. Cyclic redundancy checking is a method of checking for errors in data that has been transmitted on a communications link. A sending device applies a 16- or 32-bit polynomial to a block of data that is to be transmitted and appends the resulting cyclic redundancy code (CRC) to the block. The receiving end applies the same polynomial to the data and compares its result with the result appended by the sender. If they agree, the data has been received successfully. If not, the sender can be notified to resend the block of data.

Ethernet uses a 32-bit CRC, while Modbus uses a 16-bit CRC for error detection and correction.

Checksums and parity bits (a special case of a checksum) are calculated by various methods of binomial addition of words within a message, not with more complex polynomials.

Error correcting code is redundant data added to the communications message that allows the correction of errors, within the limits of the code used, without requiring retransmission of data.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.

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