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# AutoQuiz: What Is the Maximum Level of Precision Available Using Modbus?

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AutoQuiz is edited by Joel Don, ISA's social media community manager.

Today's automation industry quiz question comes from the ISA Certified Automation Professional certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program. The following question comes from the CAP study guide, Performance Domain III, System Design − design, specify, and procure the hardware/software used in the system.

Using Modbus, what is the MAXIMUM level of precision available?

a) 8 bits
b) 16 bits
c) 24 bits
d) 32 bits
e) none of the above

Common Modbus uses two 8-bit bytes for each data register.

Two × 8 bits = 16 bits

To achieve more precision, special programming or a specialized version of Modbus would be necessary. Eight bits are less precise than 16 bits. 24 bits and 32 bits require uncommon means to achieve.

Recall a "bit" is a binary digit, taking a value of either 0 or 1. Binary digits are a basic unit of information storage and communication in digital computing and digital information theory.

The correct answer is B, 16 bits.

Reference: Bela Liptak, Instrument Engineers Handbook - Volume 3 (Fourth Edition). Process Software and Digital Networks.

###### Joel Don
Joel Don is the community manager for ISA and is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.