March 2009
ISA Certified Automation Professional (CAP) program
Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.
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
CAP question
Using Modbus, what is the MAXIMUM level of precision available?
A. 8-bits
B. 16-bits
C. 24-bits
D. 32-bits
CAP answer
The correct answer is B, 16-bits.
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 Modus 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.
Reference: Bela Liptak, Instrument Engineers Handbook - Volume 3 (3rd Edition). Process Software and Digital Networks, ISA, 2002
Read questions answered by our experts or join the email list.
