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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Programmable Logic Controllers for Industrial Maintenance › Lesson 4
MFG-INDU16-06 · Lesson 4 · 13 clock hours · lab

Analog I/O Wiring and Signal Scaling

Objective. Students wire a 4–20 mA analog current-loop transmitter and a 0–10 VDC analog output to an analog I/O module and scale the raw integer values to engineering units using the manufacturer's scaling function or a linear calculation.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Analog I/O Wiring and Signal Scaling

Why Analog Signals Matter

Most real-world process variables—pressure, temperature, flow, level—are continuous quantities, not simple on/off states. A PLC handles these through its analog I/O modules. The two signal types you will work with most often in industrial and marine manufacturing are the 4–20 mA current loop (for inputs from transmitters) and the 0–10 VDC voltage signal (often used for analog outputs to drives or control valves).

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The 4–20 mA Current Loop

The 4–20 mA standard is the dominant process-instrumentation signal worldwide. Current is used rather than voltage because current does not change along the length of a wire, making the signal immune to resistive voltage drops over long cable runs—common in large plants and on ships.

Why 4 mA, not 0 mA, represents zero span:

Using 4 mA as the live-zero serves two purposes. First, any reading below 4 mA (such as 0–3 mA) immediately flags a wiring fault, broken transmitter, or loss of power—it is a built-in fault-detection feature. Second, the loop power supply can confirm the instrument is powered and connected at all times. A true 0 mA signal would be indistinguishable from a dead wire.

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Loop-Powered (Two-Wire) Transmitters

A two-wire transmitter has no separate power supply wires. The transmitter draws its operating power from the loop current itself (typically a 24 VDC supply in series with the loop). The circuit path is: 24 VDC supply (+) → transmitter (+) → transmitter (−) → analog input module → supply (−). The module measures the current flowing through the loop. Always verify polarity before energizing—reversing the wires will damage the transmitter or produce an incorrect reading.

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Analog Module Raw Counts

The analog input module converts loop current to a raw integer (counts) using its internal analog-to-digital converter (ADC). The count range depends on the module's resolution. A common convention for a 12-bit module is 0–4095 counts for the full signal range; some manufacturers use 0–32767 (15-bit) or other ranges. Always consult the module's data sheet.

For a module scaled 0–4095 over 4–20 mA:

Record the actual raw counts at each test point; small offsets are normal and are corrected by calibration.

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Linear Scaling: Converting Counts to Engineering Units

A linear relationship exists between raw counts and engineering units. The general equation is:

y = mx + b

Where:

Many PLC manufacturers provide a built-in scaling instruction (Allen-Bradley uses SCL or SCP; other brands have equivalent blocks) that accepts the raw input, the raw minimum and maximum, and the engineering-unit minimum and maximum, then outputs the scaled value automatically. Whether you use the instruction or write the math yourself, the underlying calculation is the same linear equation.

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Analog Output: 0–10 VDC

An analog output module converts a raw integer from the PLC program to a voltage (0–10 VDC) or current (4–20 mA) signal to drive a device such as a variable-frequency drive speed reference or a control valve positioner. The scaling works in reverse: you write an engineering-unit value in the program, and a reverse calculation produces the correct raw count, which the DAC (digital-to-analog converter) converts to the output signal. Verify the output signal with a calibrated multimeter at the terminal.

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Pass Tolerance

For this lab, the scaled PSI value displayed in the program monitor must be within ±2% of full span of the calibrated reference gauge reading at 0%, 50%, and 100% of the transmitter range. Calculate your tolerance window before testing: if the transmitter spans 0–100 PSI, ±2% of span = ±2 PSI.

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Safety

Key terms10

4–20 mA current loopA two-wire process instrumentation signal standard in which 4 mA represents 0% of span and 20 mA represents 100% of span; the current signal is immune to resistive voltage drops along the cable.
Live zeroThe practice of using 4 mA (not 0 mA) as the signal minimum so that a reading below 4 mA immediately indicates a wiring fault or transmitter failure rather than a valid zero reading.
Loop-powered (two-wire) transmitterA transmitter that draws its operating power from the same two-wire loop that carries the signal current, requiring no separate power supply wires at the device.
Raw countsThe integer value output by an analog input module's analog-to-digital converter, representing the measured signal level before any engineering-unit conversion is applied.
Analog-to-digital converter (ADC)The electronic circuit inside an analog input module that converts a continuous electrical signal (current or voltage) into a discrete integer value the PLC processor can read.
Scaling (linear)A mathematical calculation (y = mx + b) that converts raw counts from an analog module to meaningful engineering units such as PSI, °F, or GPM.
SCL / SCP instructionPLC manufacturer-supplied scaling instructions (names vary by brand) that accept raw-count input along with the raw and engineering-unit ranges and output a scaled engineering-unit value automatically.
SpanThe difference between the maximum and minimum values of a transmitter's measurement range; for example, a 0–100 PSI transmitter has a span of 100 PSI.
Analog output moduleA PLC module that converts a raw integer value from the program into a proportional voltage (e.g., 0–10 VDC) or current (e.g., 4–20 mA) signal to control a field device.
Percent of span toleranceAn accuracy specification expressed as a percentage of the full measurement range rather than the reading; ±2% of a 100 PSI span equals ±2 PSI regardless of the actual reading.

Worked examples4

1. A 4–20 mA pressure transmitter is ranged 0–200 PSI. The analog input module converts 4–20 mA to 0–4095 counts. What raw count value should appear at 50% of transmitter range (100 PSI)?
  1. Identify 50% of span signal: 4 mA + 0.50 × (20 − 4) = 4 + 8 = 12 mA
  2. Determine where 12 mA falls in the count range: (12 − 4) / (20 − 4) = 8 / 16 = 0.50
  3. Apply the fraction to the count range: 0.50 × 4095 = 2047.5, rounded to 2048 counts
Answer: Approximately 2048 raw counts at 100 PSI (50% of span).
2. Using the same transmitter and module (0–200 PSI, 0–4095 counts), write the linear scaling equation (y = mx + b) and calculate the PSI value when the module reads 3000 counts.
  1. Calculate slope m: m = (EU_max − EU_min) / (Count_max − Count_min) = (200 − 0) / (4095 − 0) = 200 / 4095 ≈ 0.04884 PSI/count
  2. Calculate offset b: b = EU_min − (m × Count_min) = 0 − (0.04884 × 0) = 0
  3. Apply the equation: y = 0.04884 × 3000 + 0 = 146.52 PSI
Answer: Approximately 146.5 PSI when the module reads 3000 counts.
3. The lab pass tolerance is ±2% of span for a 0–200 PSI transmitter. At the 50% test point (100 PSI reference gauge), what is the acceptable displayed-value range in PSI?
  1. Calculate ±2% of span: 0.02 × 200 PSI = 4 PSI
  2. Apply tolerance to the reference reading: 100 − 4 = 96 PSI (lower limit); 100 + 4 = 104 PSI (upper limit)
Answer: The scaled display must read between 96 PSI and 104 PSI to pass.
4. A 0–10 VDC analog output module uses a raw count range of 0–32767 to represent 0–10 VDC. A program value of 16384 counts is written to the output. What voltage should appear at the terminals?
  1. Calculate the fraction of full scale: 16384 / 32767 = 0.5001 ≈ 0.50 (50%)
  2. Apply to voltage range: 0.50 × 10 VDC = 5.00 VDC
Answer: Approximately 5.00 VDC at the analog output terminals.

The math in this lesson2

Learners compute the ±2 percent of span acceptance range at the 100 PSI test point.
7.RP.3F-LE.2
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates loop-powered transmitter wiring, explains the 4–20 mA standard and why 4 mA represents zero span rather than 0 mA, using a clamp-on ammeter to show live loop current. · Students wire a pressure transmitter in a two-wire loop, read the raw integer from the PLC data table at 0%, 50%, and 100% of the transmitter range, and record all three values in their lab log. · Students write or enter a linear scaling calculation (y = mx + b or manufacturer SCL/SCP instruction) in the program to convert raw counts to pressure in PSI and verify the displayed value against a calibrated reference gauge — pass tolerance is ±2% of span.
Materials: Analog I/O module, 4–20 mA pressure transmitter with test pump, calibrated reference gauge, 24 VDC loop power supply, clamp-on ammeter, multimeter, programming laptop, scaling calculation worksheet
Check: Scaled engineering-unit value displayed in program monitor must be within ±2% of the reference gauge reading at all three test points; student documents results in lab log.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Process Instrumentation and Measurement
1. Why does the 4–20 mA standard use 4 mA as the zero-span value rather than 0 mA?
Electrical Wiring and Instrumentation
2. A loop-powered (two-wire) transmitter requires a 24 VDC supply. Where does the transmitter obtain its operating power?
PLC Analog I/O
3. A 4–20 mA transmitter is ranged 0–100 PSI. The analog input module converts the signal to 0–4095 counts. What is the expected raw count when the process is at exactly 0 PSI?
Mathematical Scaling and Calculation
4. Using a linear scaling equation for a 0–150 PSI transmitter with a 0–4095 count range, what is the slope (m) in PSI per count?
Safety and Hazard Control
5. Before changing any wiring connections inside a PLC panel, which safety procedure must be followed according to OSHA standards?
Quality and Process Measurement
6. The lab pass tolerance is ±2% of span. The transmitter is ranged 0–50 PSI. The reference gauge reads 25 PSI. Which displayed PLC value would FAIL the check?
Fault Diagnosis and Troubleshooting
7. A technician measures 2.1 mA in a 4–20 mA loop while the process is running normally. What does this most likely indicate?
Test Equipment and Measurement
8. Which instrument should a technician use to verify the actual current flowing in a 4–20 mA loop without breaking the circuit?
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