4-20 mA calculator: scaling, 0-10 V shunt and PLC input check

Analog inputs of Industrial Shields PLCs for 4-20 mA

All four families have 0-10 Vdc analog inputs. What changes is the resolution and, on the M-Duino and the Ardbox, the 39 kΩ input impedance given in the datasheet. Values with a 500 Ω shunt:

ESP32 PLC 14: inputs AI0.0 and AI0.1 read 4-20 mA directly in 4-20 mA mode, with no shunt. This calculator covers the 0-10 V inputs.

On the M-Duino 21+, the ESP32 PLC 21+ and the Raspberry PLC 21, the analog inputs are I0.7 to I0.12; larger models add more zones with the same pattern. On the Ardbox Analog they are I0.2 to I0.9.

With an ideal input and 500 Ω, 20 mA lands exactly on 10 V, so the ESP32 PLC and the Raspberry PLC have no margin above 20 mA: 470 Ω gives that margin.

How to connect a 4-20 mA transmitter to a 0-10 V PLC input

1. Choose the resistor. 500 Ω for a 0-10 V input, or 470 Ω if you need to see the NAMUR NE 43 high fault. Use 1 % or better and at least 0.5 W.
2. Wire the loop. 2-wire: +24 V to the + of the transmitter, the - of the transmitter to the analog input, and the resistor between the input and 0 V. 3-wire and 4-wire: the output of the transmitter to the input, its 0 V to the common 0 V.
3. Check the loop voltage. At 20 mA the supply must still leave the minimum voltage of the transmitter after the resistor and the cable.
4. Scale and check faults in the program. Convert the reading to mA, check the NAMUR NE 43 limits, then scale to units. Check 4, 12 and 20 mA with a loop calibrator.

Example code: 4-20 mA on an M-Duino

This is the code the calculator gives for a 0 to 10 bar transmitter on input I0.7 of an M-Duino with 500 Ω. Change the values above and copy your version from the calculator.

// 4-20 mA transmitter on M-Duino 21+ input I0.7 (M-Duino: 0-10 Vdc, 10 bit)
// Shunt resistor of 500 ohm between I0.7 and GND turns the loop current into a voltage.
// Generated by the Industrial Shields 4-20 mA calculator: https://www.industrialshields.com/plc-solution-library/tools/4-20ma-calculator
// Check the values on your hardware with a loop calibrator before you use them.
// Arduino IDE with the Industrial Shields AVR boards package. Select your M-Duino model.

const int AI_PIN = I0_7;   // analog input used for the loop
const float ADC_MAX   = 1023.0;   // 10 bit, from the M-Duino Family datasheet
const float AI_RANGE  = 10.0;     // input range in volts
const float R_SHUNT   = 500.0;    // shunt resistor in ohm
const float R_INPUT   = 39000.0;    // input impedance in ohm (0 = not known, treated as ideal)
const float R_EQ = (R_INPUT > 0.0) ? R_SHUNT * R_INPUT / (R_SHUNT + R_INPUT) : R_SHUNT;

const float PV_MIN = 0.000;   // value at 4 mA (bar)
const float PV_MAX = 10.000;   // value at 20 mA (bar)

// NAMUR NE 43 limits in mA
const float MA_FAULT_LOW  = 3.6;   // 3.6 mA or less: broken wire or low fault signal
const float MA_RANGE_LOW  = 3.8;   // 3.6 to 3.8 mA: under range
const float MA_RANGE_HIGH = 20.5;  // 20.5 to 21 mA: over range (saturated)
const float MA_FAULT_HIGH = 21.0;  // 21 mA or more: high fault signal

const int N_SAMPLES = 16;

float readCounts() {
  uint32_t sum = 0;
  for (int i = 0; i < N_SAMPLES; i++) {
    sum += analogRead(AI_PIN);
  }
  return (float)sum / N_SAMPLES;
}

float countsToMilliamps(float counts) {
  float volts = counts * AI_RANGE / ADC_MAX;
  return volts / R_EQ * 1000.0;
}

float milliampsToValue(float mA) {
  float pv = PV_MIN + (mA - 4.0) * (PV_MAX - PV_MIN) / 16.0;
  if (PV_MIN < PV_MAX) {
    if (pv < PV_MIN) pv = PV_MIN;
    if (pv > PV_MAX) pv = PV_MAX;
  }
  return pv;
}

void setup() {
  Serial.begin(115200);
  pinMode(AI_PIN, INPUT);
}

void loop() {
  float counts = readCounts();
  float mA = countsToMilliamps(counts);
  float pv = milliampsToValue(mA);

  Serial.print(mA, 2);
  Serial.print(" mA  ");
  if (MA_FAULT_LOW >= mA) {
    Serial.println("LOOP FAULT: broken wire or low fault signal");
  } else if (mA >= MA_FAULT_HIGH) {
    Serial.println("HIGH FAULT SIGNAL from the transmitter");
  } else if (counts >= ADC_MAX - 0.5) {
    Serial.print(pv, 2);
    Serial.println(" bar  (input at full scale: the current can be higher)");
  } else {
    Serial.print(pv, 2);
    Serial.print(" bar");
    if (MA_RANGE_LOW > mA) Serial.println("  (under range)");
    else if (mA > MA_RANGE_HIGH) Serial.println("  (over range, saturated)");
    else Serial.println();
  }
  delay(500);
}