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This post was authored by Joel Don, ISA's social media community manager.

This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) 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.

A set of new, 4-20mA gage pressure transmitters has been wired to an analog input card in an existing PLC system, and you are tasked with scaling the raw input values in the PLC logic. This analog card has the following characteristics:

• Unsigned, 14-bit analog data format
• Eight (8) channels, 0-20mA inputs
• Single-ended inputs
• No channel diagnostics
• Data stored internally in integer “counts,” from 0 up to the maximum 14-bit value

After reviewing these specifications, you determine the correct equation to program into the PLC to determine the current process values from the new transmitters is (where Engineering Units Value at 20 mA = EU100% and Engineering Units Value at 4 mA = 0 psig):

a) Analog Value in Eng Units=[EU100%-(Raw Counts-3277)] ÷ 4096
b) Analog Value in Eng Units=[(Raw Counts-819) × EU100%] ÷ 6048
c) Analog Value in Eng Units=[(Raw Counts-3277) × EU100%] ÷ 13106
d) Analog Value in Eng Units=[(Raw Counts+1637) ÷ EU100%] × 16384
e) none of the above

The correct answer is C. To scale the analog input, calculate the “% of the raw counts range” that is represented by the analog input, and multiply by the EU100% range, since the EU0% value is 0.

For a 14-bit input with no sign bit or diagnostics bit, the maximum number of counts is (2^14 – 1), or 16383. Since the analog input card raw counts are for a 0-20mA signal, we need to determine the number of raw counts that represents a 4-20mA signal. 4mA is 20% of the 0-20mA range, so the raw counts that correspond to a 4-20mA signal: (20% x 16383) = 3277 counts at 4mA and 16383 counts at 20mA. Therefore, the span of raw counts is (16383 – 3277) = 13106 counts.

Therefore, the percent of range of the current analog value is = (Raw Counts – 3277) / 13017. Multiplying by EU100% gives us the scaled analog input value.

Reference: Nicholas Sands, P.E., CAP and Ian Verhappen, P.Eng., CAP., A Guide to the Automation Body of Knowledge. To read a brief Q&A with the authors, plus download a free 116-page excerpt from the book, click this link.