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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 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


Click Here to Reveal the Answer

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 Modus 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 (4th Edition), Process Software and Digital Networks

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).

The correct answer is B, 16 bits.



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