---
title: "PT100 on X-RTD4: Motor-Winding and Transformer Temperature with 3-Wire Compensation"
description: "## Introduction This tutorial connects a 3-wire PT100 probe to a NORVI X-RTD4 expansion module and reads it with a NORVI X-CPU-ESPS3-X1 over I^2C. The X-RTD4 digitises the probe with a MAX31865 RTD-to-digital converter and an on-board STM32 microcontroller. It…"
url: "https://norvi.io/docs/pt100-on-x-rtd4-3-wire-temperature-monitoring/"
updated: "2026-11-05"
category: NORVI X / NORVI X TUTORIALS
---

# PT100 on X-RTD4: Motor-Winding and Transformer Temperature with 3-Wire Compensation

## Introduction

This tutorial connects a 3-wire PT100 probe to a NORVI X-RTD4 expansion module and reads it with a NORVI X-CPU-ESPS3-X1 over I^2C. The X-RTD4 digitises the probe with a MAX31865 RTD-to-digital converter and an on-board STM32 microcontroller. It returns a linearised temperature, the resistance and a fault byte in a CRC-protected frame. The sketch requests the configured channels once per second, validates every frame, and shows the temperature, resistance and fault state of each channel on the controller’s TFT and on Serial.

The same method suits motor windings, transformer windings, bearing housings, pump casings and any zone that stays inside the rating of the probe.

## System Architecture

The system forms a short signal chain from the winding probe to the operator display. The hardware blocks and the data-processing logic are illustrated below.

*Signal path from the winding probe to the operator display*

![Data flow and CRC validation logic from the RTD channels to the display](https://norvi.io/wp-content/uploads/2026/11/Data-flow-and-CRC-validation-logic-from-the-RTD-channels-to-the-display.png)
Data flow and CRC validation logic from the RTD channels to the display

### Signal flow

1. **Sensing.** The PT100 element rises from 100 Ω at 0 °C to about 138.5 Ω at 100 °C and 168.5 Ω at 180 °C.
2. **Multiplexing and conversion.** An analog multiplexer connects one of the four channels at a time to a single MAX31865. The converter excites the element and returns a 15-bit reading.
3. **Bridging.** The on-board STM32 handles the multiplexing and the converter’s SPI interface, then answers the CPU on the shared I^2C bus. This offloads the conversion work from the main controller.
4. **Processing.** The ESP32-S3 requests up to two channels per transaction, checks the CRC of every 12-byte frame, and stores temperature, resistance and fault per channel.
5. **Indication.** Changed values are redrawn on the TFT, and one line per channel is printed on Serial.

### Hardware components

| **Component** | **Role** | **Key details** |
| --- | --- | --- |
| NORVI X-CPU-ESPS3-X1 | Controller | ESP32-S3, 16 MB flash, 2 MB QSPI PSRAM, 2.0 in ST7789 TFT (240 × 320), 24 V DC supply |
| NORVI X-RTD4 | RTD input | 4 RTD channels, MAX31865 (15-bit) shared through a multiplexer, STM32F103C8T6 bridge, PT100/PT1000 by DIP switch, I^2C address by DIP switch (default 0x3F) |
| PT100 probe | Field sensor | 3-wire, R~0 = 100 Ω, Class A or B; for windings, an embedded sensor or a probe in a pocket, as the equipment manufacturer specifies |
| V DC supply | Power | Feeds the X-CPU; the X-RTD4 takes its power through the expansion port (80 mA at 24 V) |

### Pin allocation

| **Function** | **GPIO** | **Notes** |
| --- | --- | --- |
| I^2C (SDA / SCL) | IO8 / IO9 | The only signals the X-RTD4 uses on the expansion port, apart from power. Shared with the X-CPU’s own devices (touch 0x15, PCA9536 0x41, RTC 0x68, PCA9539 0x75) |
| SPI (MOSI / MISO / SCLK) | IO11 / IO13 / IO12 | Display bus. The sketch calls SPI.begin(SCLK, MISO, MOSI) |
| TFT (CS / DC / RST) | IO45 / IO46 / IO47 | ST7789 panel, driven through LovyanGFX |
| X-RTD4 address | 0x3F | All address DIP switches OFF. Matches RTD_SLAVE_ADDR in the sketch |

## Hardware and Wiring

### X-RTD4 at a glance

| **Parameter** | **Value** |
| --- | --- |
| Inputs | 4 RTD channels, terminals 1 to 8 (RTD1+ / RTD1− to RTD4+ / RTD4−), terminal 9 GND |
| Converter | MAX31865, 15-bit, with automatic fault detection |
| Interface to CPU | I^2C through the expansion bus, address set by a 4-way DIP switch: 0x3F (all OFF) down to 0x30 (all ON) |
| RTD type switch | ON = PT100, OFF = PT1000 |
| Supply | 24 V DC through the expansion port, 80 mA at 24 V |
| Environment | −40 to +85 °C, IP20, DIN rail, 23 × 81 × 104 mm |
| Indicators | One LED per channel (1 to 4), an A LED and a power LED |

### Connect the module and the sensor

1. Power off the whole system.
2. Mount the X-RTD4 on the DIN rail beside the X-CPU and seat it firmly on the expansion bus connector.
3. Set the address DIP switches all OFF (0x3F) and the RTD type switch to **ON** for PT100.
4. Identify the probe leads with a multimeter (see Section [3.3](#sec:practice)). The two leads of the matched pair read about 0 Ω to each other.
5. Wire the probe to channel 1 as in Table [[tab:wiring]](#tab:wiring) and Figure [3](#fig:wiring).
6. Connect the 24 V DC supply to the POWER terminals of the X-CPU. The RS-485 terminals are not used.
7. Power on and connect the USB cable.

| **Probe lead** | **X-RTD4 terminal** | **Function** |
| --- | --- | --- |
| Single lead (red) | 1 – RTD1+ | Positive input, channel 1 |
| One lead of the matched pair (black) | 2 – RTD1− | Negative input, channel 1 |
| Other lead of the matched pair (black) | 9 – GND | Third lead used for the 3-wire connection |

![Wiring: 24 V supply to the X-CPU, and the 3-wire PT100 on channel 1 of the X-RTD4 (red to terminal 1, black to terminal 2, second black to GND)](https://norvi.io/wp-content/uploads/2026/11/24-V-supply-to-the-X-CPU-and-the-3-wire-PT100-on-channel-1-of-the-X-RTD4-red-to-terminal-1-black-to-terminal-2-second-black-to-GND.png)
Wiring: 24 V supply to the X-CPU, and the 3-wire PT100 on channel 1 of the X-RTD4 (red to terminal 1, black to terminal 2, second black to GND)

### Sensor wiring practice

A PT100 is passive and has no polarity, so it needs no supply of its own.

- Use three leads of the same gauge and length. The compensation assumes the leads are equal, so unequal leads leave an error.
- Check the probe before wiring: about 100 Ω at 0 °C, about 110 Ω at room temperature, and about 0 Ω between the matched pair.
- Use shielded, twisted cable rated for the temperature it passes through. Ground the shield at one end only.
- Keep sensor cables short and away from motor, VFD, AC and relay wiring. Cross power cables at 90° if you must cross them.
- Tighten every terminal. A loose lead gives a fault or a reading that drifts.
- Channels 2 to 4 are wired the same way on terminals 3 and 4, 5 and 6, and 7 and 8. GND (terminal 9) is shared.
- Do not connect or disconnect the module while the system is powered.

## How a PT100 Is Read with 3-Wire Compensation

A PT100 is a platinum resistor with 100 Ω at 0 °C and a slope of about 0.385 Ω/°C. The converter measures its resistance, so any resistance in the cable adds to the reading. With two wires, 1 Ω of cable reads as about 2.6 °C. A 20 m run of 0.5 mm^2 copper has roughly 0.69 Ω per lead, which is about 3.6 °C of error for the two leads in the measuring loop. That matters on a motor terminal box or a transformer cabinet a few metres from the winding.

A third lead removes most of this. It lets the converter measure the lead resistance and subtract it, leaving the element resistance. This works only when the three leads are matched.

| **Parameter** | **PT100** | **PT1000** |
| --- | --- | --- |
| Resistance at 0 °C | 100 Ω | 1000 Ω |
| Change per °C | about 0.385 Ω | about 3.85 Ω |
| Effect of lead resistance | Higher | Lower |
| Signal strength | Lower | Higher |
| RTD type switch on X-RTD4 | ON | OFF |

The X-RTD4 user guide shows each channel as a 2-wire input with GND as a shared reference. This tutorial connects the third lead to the GND terminal as shown in Figure [3](#fig:wiring). Do not assume the lead is being compensated. Run the lead-resistance test in Section [8.3](#sec:leadtest) once, with the probe replaced by a resistor, and record the result.

## Convert Resistance to Temperature

The sketch does not linearise anything. The X-RTD4 returns the temperature in °C and the resistance as 32-bit floats, and the sketch displays them as received. For reference, the PT100 characteristic above 0 °C (IEC 60751) is

R(T)=R~0 (1+A T+B T^2),  R~0=100 Ω, A=3.9083×10^−3 ^∘C^−1, B=−5.775×10^−7 ^∘C^−2.

Use Table [[tab:rt]](#tab:rt) to check readings during commissioning. The tolerance columns are the IEC 60751 limits for the probe itself, ±(0.15+0.002 |T|) °C for Class A and ±(0.30+0.005 |T|) °C for Class B.

| **Temp. (°C)** | **Ideal R (**Ω**)** | **Class A (**±**°C)** | **Class B (**±**°C)** | **Typical use** |
| --- | --- | --- | --- | --- |
|  | 100.00 | 0.15 | 0.30 | Ice-water check |
|  | 109.73 | 0.20 | 0.43 | Room temperature |
|  | 119.40 | 0.25 | 0.55 |  |
|  | 138.51 | 0.35 | 0.80 |  |
|  | 149.83 | 0.41 | 0.95 | Class B insulation limit |
|  | 159.19 | 0.46 | 1.08 | Class F insulation limit |
|  | 168.48 | 0.51 | 1.20 | Class H insulation limit |

In the photograph (Figure [4](#fig:photo)) the display shows 32.51 °C and 1126.44 Ω. A PT100 at 32.51 °C is 112.64 Ω, so the Res value is ten times the element resistance, while the temperature is correct. Treat the temperature as the primary reading for PT100 and divide Res by 10 if you need ohms.

## Motor Windings and Transformers

### Probe placement

- Use the winding sensors the manufacturer fitted, or the pocket or thermowell the manufacturer specifies. A probe taped to the outside of a casing reads the casing, not the hot spot.
- For a three-phase machine, read one probe per phase or per winding on channels 1 to 3, and use channel 4 for a bearing, core or oil pocket.
- Route all the probe cables the same way and keep them matched in length and gauge.

### Setting limits

Take the limits from the nameplate and the manufacturer’s documents, not from this guide. The insulation thermal class sets the ceiling for the winding temperature (IEC 60085).

| **Class** | **Maximum (°C)** | **Practical use** |
| --- | --- | --- |
| B | 130 | Older and smaller machines |
| F | 155 | Common industrial motors and dry-type transformers |
| H | 180 | High-temperature duty |

Set a warning point below the trip point. Remember that a sensor sitting in or near the winding reads lower than the hottest spot, so apply the margin the equipment manufacturer recommends. The tested sketch only displays values and has no alarm. Section [7](#sec:code) shows how to add a simple colour threshold.

### Faults and communication failures

| **Condition** | **What you see** | **Meaning and action** |
| --- | --- | --- |
| Normal | Fault: 0 | The converter reports no fault. The temperature is valid |
| Converter fault | Fault: non-zero | The converter’s automatic fault detection flagged the channel, typically an open or shorted probe or lead. Do not use the temperature. Check the wiring |
| Bad frame | CRC ERROR on Serial | The frame is discarded and the last good value stays. Check the I^2C wiring and seating |
| No reply | NO RESPONSE and the address on the TFT | Three scans in a row returned no valid frame. Check seating, power and the address switches |

### Reading the screen

| **Row** | **Colour** | **Content** |
| --- | --- | --- |
| NORVI RTD MASTER | Yellow | Title |
| RTD Type | Cyan | PT100 or PT1000, as set in the sketch |
| Ch n : Temp | White and green | Channel number and temperature in °C |
| Res | Orange | Resistance reported by the module (see the note in Section [5](#sec:convert)) |
| Fault | Red | Fault byte, 0 when healthy |
| NO RESPONSE | Red and white | Shown instead of old values when the module stops answering |

## Arduino Code for the ESP32 PT100 Monitor

### Set up the IDE

- Select **ESP32S3 Dev Module**, then set USB CDC On Boot to **Enabled**, Flash Size to **16MB (128Mb)** and PSRAM to **QSPI PSRAM**. Without USB CDC On Boot the Serial Monitor stays empty.
- Install **LovyanGFX** from the Library Manager. Wire and SPI come with the ESP32 core. The X-RTD4 speaks a framed I^2C protocol, so no sensor library is needed.
- The sketch starts as PT1000 on channel 1. For this tutorial change the three configuration lines to PT100 on all four channels:

```cpp

                uint8_t rtdType = 0;                      // 0 = PT100 | 1 = PT1000
uint8_t channelsToRead[4] = {1, 2, 3, 4};
uint8_t numChannels = 4;
```

The values are held in RAM only, so the sketch returns to these defaults after every reset.

### How the sketch talks to the X-RTD4

Every scan sends a request to address 0x3F, waits 15 ms, then reads 12 bytes per channel. A request carries at most two channels, so four channels need two transactions.

| **Request byte** | **Content** |
| --- | --- |
|  | Command, 0x01 |
|  | RTD type: 0 = PT100, 1 = PT1000 |
| , 3 | Channel numbers (1 to 4), one or two |

| **Reply byte** | **Content** |
| --- | --- |
|  | Not used by the sketch |
|  | Channel number |
| to 5 | Temperature in °C, 32-bit float |
| to 9 | Resistance, 32-bit float |
|  | Fault byte (0 = no fault) |
|  | CRC-8 of bytes 0 to 10 (reflected polynomial 0x8C, initial value 0) |

### What the main parts of the code do

The complete sketch is in the repository listed under Resources. In order:

- **Settings.** The address (RTD_SLAVE_ADDR), RTD type, channel list and the data arrays for temperature, resistance and fault.
- **CRC.** crc8() checks every frame. A bad frame is skipped, so the last good value stays on screen.
- **Reading.** readRTD() sends the request, waits 15 ms, collects the frames, checks the CRC, stores the values and prints one Serial line per channel.
- **Failure handling.** After two failed transmissions the sketch clears the bus by clocking SCL up to nine times and restarts I^2C. After three scans in a row without a valid frame the display shows NO RESPONSE.
- **Timing.** The loop never blocks. It retries every 100 ms until the first valid frame and after any failed transmission, then reads once per second.
- **Display.** Each text row is cached, and only rows whose content changed are cleared and redrawn, so the screen does not flicker.
- **Serial configuration.** Send C in the Serial Monitor, then one line of RTD type and channels: 100,1 (PT100, channel 1), 100,1-4 (all four) or 100,1,3 (channels 1 and 3). Invalid input leaves the configuration unchanged.

### Optional: colour threshold

This snippet is not part of the tested sketch. Place it above drawValues(), set the limits from the equipment documents, and replace tft.setTextColor(TFT_GREEN); in the temperature row with tft.setTextColor(tempColor(rtdTemp[ch-1], rtdFault[ch-1]));.

```cpp

                #define TEMP_WARN 140.0f   // set from the nameplate / manufacturer
#define TEMP_TRIP 155.0f

uint16_t tempColor(float t, uint8_t fault)
{
  if (fault)          return TFT_MAGENTA;   // invalid reading
  if (t >= TEMP_TRIP) return TFT_RED;
  if (t >= TEMP_WARN) return COLOR_ORANGE;
  return TFT_GREEN;
}
```

## Commissioning and Test

### Commissioning checklist

- **Supply.** The 24 V DC supply is stable and wired to the X-CPU POWER terminals. The module’s power LED is on.
- **Switches.** Address DIP switches all OFF, RTD type switch ON, sketch set to type 0 (PT100). The switch and the sketch must agree.
- **I****^2****C scan.** The scan below lists 0x3F together with 0x15, 0x41, 0x68 and 0x75 from the X-CPU.
- **Channel LED.** The LED of each channel in use lights while it is being read.

To scan the bus, call this from setup() after Wire.begin():

```cpp

                void i2cScan() {
  for (byte a = 1; a < 127; a++) {
    Wire.beginTransmission(a);
    if (Wire.endTransmission() == 0) Serial.printf(" found 0x%02Xn", a);
  }
}
```

### Resistor substitution

Replace the probe with a precision 100 Ω resistor between terminals 1 and 2, and link terminal 2 to terminal 9 so that the resistor sees the same connection as the probe. The display should read about 0 °C. A 138.5 Ω resistor should read about 100 °C.

### Lead-resistance test

This test shows whether your unit compensates the leads. Keep the 100 Ω resistor from the previous step as the element and add a 2.0 Ω resistor in series with each of the three leads, as in Table [[tab:leadtest]](#tab:leadtest).

| **Connection** | **Terminal** | **Note** |
| --- | --- | --- |
| Element end A through 2.0 Ω | 1 – RTD1+ | Simulates the single lead |
| Element end B through 2.0 Ω | 2 – RTD1− | First lead of the matched pair |
| Element end B through 2.0 Ω | 9 – GND | Second lead of the matched pair |

Compare the reading with the one from the previous step.

- If it stays within about 1 °C of the previous reading, the module compensates the leads.
- If it rises by about 10 °C (4 Ω of added loop resistance at 0.385 Ω/°C), the module reads the channel as a 2-wire input. Keep the probe cable short, and do not rely on 3-wire compensation.

### Probe checks

- **Ice water.** With the probe in melting ice water the reading settles near 0 °C within the probe tolerance.
- **Warm by hand.** The temperature rises steadily when you hold the probe.
- **Open lead.** Disconnect the red lead. Fault should become non-zero, and the temperature must not be used.

### Expected output

Serial prints one line per channel and per scan, at 115200 baud. The example below matches the photograph, with the probe in room air:

```
Ch 1 Temp: 32.51 Res: 1126.44 Fault: 0
```

![Working setup: X-CPU (left), X-RTD4 with the 3-wire PT100 on channel 1, and the 24 V DIN-rail supply. The display shows PT100, 32.51 °C and Fault 0](https://norvi.io/wp-content/uploads/2026/11/X-CPU-left-X-RTD4-with-the-3-wire-PT100-on-channel-1-and-the-24-V-DIN-rail-supply.-The-display-shows-PT100-32.51-°C-and-Fault-0.png)
Working setup: X-CPU (left), X-RTD4 with the 3-wire PT100 on channel 1, and the 24 V DIN-rail supply. The display shows PT100, 32.51 °C and Fault 0

## Troubleshooting

| **Symptom** | **Likely cause** | **Remedy** |
| --- | --- | --- |
| Serial Monitor empty | USB CDC On Boot disabled or wrong port | Enable USB CDC On Boot, reselect the port, set 115200 baud, press reset |
| Display blank | Wrong board settings, or display pins changed | Use the settings in Section [7](#sec:code). Keep CS 45, DC 46, RST 47 in the LGFX class |
| NO RESPONSE | Module not seated, no power, or address mismatch | Power off and reseat. Check the module’s power LED. Match RTD_SLAVE_ADDR to the address switches |
| Temperature far from expected | RTD type switch and sketch disagree | Set the switch to ON for PT100 and rtdType = 0, or OFF and 1 for PT1000 |
| Res is ten times high on PT100 | Resistance scaling on PT100 | Use the temperature, divide Res by 10 for ohms (Section [5](#sec:convert)) |
| Fault non-zero | Open or shorted lead, loose terminal | Check terminals 1, 2 and 9 and measure the probe with power off |
| Reading a few degrees high | Unequal or long leads, or no compensation | Use matched leads and run the test in Section [8.3](#sec:leadtest) |
| CRC ERROR | Noise or poor seating on the I^2C bus | Reseat the module, keep it away from motor cabling, shorten any extension |
| Setting lost after reset | Configuration is held in RAM | Edit the defaults in the sketch (Section [7](#sec:code)) |

Disconnect all power before wiring and let a qualified electrician install the system. This monitor provides indication only. Do not use it as a motor or transformer protection device.

## FAQ

 ****Can an ESP32 read a PT100 directly?****

It can read a voltage, but its ADC is noisy and non-linear, and it cannot excite the element or remove cable resistance. The X-RTD4 does the excitation and conversion and hands the CPU a clean temperature over I^2C.

  ****PT100 or PT1000?****

Both work. PT1000 gives about ten times the signal and is less affected by cable resistance, so it suits long runs. Match the RTD type switch and the sketch to the probe.

  ****Why does 3-wire matter for a motor winding?****

The terminal box can be several metres from the winding. Cable resistance adds to the reading at about 2.6 °C per ohm. A matched third lead lets the converter subtract it, provided the compensation is active on your unit (Section [8.3](#sec:leadtest)).

  ****Can I read more than one probe?****

Yes. Send C, then 100,1-4 for all four channels. The channels are scanned one after another through the multiplexer.

  ****Can I use more than one X-RTD4?****

Yes, with a different address on each module through the DIP switches. The sketch holds a single RTD_SLAVE_ADDR, so a second module needs a second address and request loop.

  ****Is this a protection relay?****

No. It displays and logs temperature. Use a certified protection device for tripping.

## Conclusion

You now have a working PT100 monitor: a 3-wire probe on the X-RTD4, converted by the module, read by the X-CPU over I^2C with a CRC check, and shown on the TFT and on Serial. Run the commissioning tests once, record the lead-resistance result, and set the limits from the equipment documents. To go further, add the colour threshold, log the readings to the SD card, or publish them over MQTT.

## Resources

- NORVI X-RTD4 datasheet and user guide:
  [https://norvi.io/docs/norvi-x-rtd4-datasheet/](https://norvi.io/docs/norvi-x-rtd4-datasheet/)
- NORVI-X-Version-02 repository (libraries, expansion module examples):
  [https://github.com/NORVIControllers/NORVI-X-Version-02](https://github.com/NORVIControllers/NORVI-X-Version-02)
- This tutorial’s code: Application Guides / pt100-on-x-rtd4 in the NORVI-X-Version-02 repository
- MAX31865 RTD-to-digital converter datasheet (Analog Devices)
- Technical support: support@norvi.io