RTD Data Acquisition 8-Layer Stackable HAT for Raspberry Pi

Overview

Eight-channel RTD data acquisition HAT for Raspberry Pi supporting both PT100 and PT1000 sensors, selected in software — no jumpers required. Uses 24-bit delta-sigma ADCs for factory accuracy of 0.1%, calibratable to 0.01% in the field. Eight-layer stackable for up to 64 RTD channels per Raspberry Pi. Includes RS485/MODBUS, hardware watchdog, and programmable status LEDs. Compatible with all Raspberry Pi from Zero to 5.

• PT100 sensors: −200°C to 765°C range · PT1000 sensors: −50°C to 500°C range
• Software-selectable sensor type — no jumpers or hardware changes required (V7+)
• 24-bit delta-sigma ADCs · Factory accuracy 0.1% · Field calibration to 0.01%
• Maximum acquisition speed: 40 cps · 3-wire circuit design compensates for lead resistance
• RS485/MODBUS port · Hardware watchdog · Eight programmable threshold LEDs
• Eight-layer stackable — up to 64 RTD channels · Uses only I2C — 24 GPIO pins remain free
• 5V powered from Raspberry Pi or own connector · Resettable fuse · Compatible with all Raspberry Pi Zero through 5

Works with Open Automation Software

Compatible with widely used tools for control, monitoring, and system integration.

INTERFACES AND I/O

DESCRIPTION

The RTD Data Acquisition HAT turns Raspberry Pi into a compact and accurate temperature acquisition system. It supports up to eight PT100 or PT1000 sensors — sensor type is selected in software with no jumpers or hardware changes required (V7+). Using 24-bit delta-sigma ADCs, the card achieves better than 0.1% factory accuracy, calibratable to 0.01% in the field using a precision 100Ω resistor. Up to eight cards stack on one Raspberry Pi for 64 RTD channels.

TECHNICAL DETAILS

CARD LAYOUT

MECHANICAL SPECIFICATIONS

The Eight RTD DAQ Card conforms to the Sequent Microsystems Industrial HAT format for Raspberry Pi. All cards in this family share the same mechanical dimensions and connector layout, and feature nine LEDs and a pushbutton located in identical positions. This standardized design simplifies system integration and allows multiple cards to be used within a common enclosure.

DIP SWITCH AND STACKING

The onboard six-position DIP switch configures both the stack address and the RS485 port behavior. Cards of different Sequent HAT types can be freely mixed in the same stack.

ELECTRICAL SPECIFICATIONS

POWER REQUIREMENTS

PLUGGABLE CONNECTORS

All RTD inputs connect to 3.5mm pitch pluggable screw terminal connectors, accepting wire gauges from 26 to 16 AWG. Connectors can be unplugged from the board for convenient field wiring and debugging without disturbing the rest of the installation.

FIELD CALIBRATION

The A/D converter measures temperature by sourcing a precise 1 mA excitation current through the RTD sensor and measuring the resulting voltage. The current source is referenced to an 820 Ω, 0.1% precision resistor, which limits the accuracy of an uncalibrated board to approximately 0.1%.

For applications requiring higher accuracy, each input channel supports two-point calibration. Using a precision 100 Ω reference resistor, calibration accuracy better than 0.01% can be achieved. For best results, select two calibration points that are as far apart as possible within the measurement range.

Calibration coefficients are stored in non-volatile flash memory and are automatically applied to all subsequent resistance and temperature measurements.

First, short the RTD input to be calibrated and issue the following command:

megartd <id> c[alibrate] <channel> 0

Next, connect a precision 100 Ω resistor (0.01% tolerance recommended) to the same input and issue:

megartd <id> c[alibrate] <channel> 100

While any precision resistor between 0 Ω and 100 Ω may be used for calibration, the best results are obtained with a resistor value as close as possible to 100 Ω, which corresponds to the upper calibration point.

The resistance-to-temperature conversion is performed using the equation:

T = (R − R₀) / (R₀ × k)

where:

T = temperature in °C
R = measured sensor resistance
R₀ = resistance at 0°C (100 Ω for PT100, 1000 Ω for PT1000)
k = 0.00385

If higher conversion accuracy is required, the software can also return the raw resistance values, allowing users to implement custom polynomial conversion algorithms.

To restore the factory calibration values for all channels, issue the command:

megartd <id> rc[alibrate]

The RESET command restores all calibration coefficients to their factory values. After a reset, the calibration procedure can be repeated at any time.

COMMUNICATION INTERFACES

HARDWARE WATCHDOG

The RTD Data Acquisition HAT includes a hardware watchdog that ensures your system automatically recovers if the Raspberry Pi software becomes unresponsive. The watchdog is disabled at power-up and activates after it receives the first reset command from the Raspberry Pi.

For a full list of watchdog commands, run: rtd -h

COMPATIBILITY

FIRMWARE UPDATE

The onboard microcontroller firmware can be updated in the field. The update tool downloads the latest firmware from Sequent Microsystems' servers and flashes it to the board over I2C.

Running the Update

~$ cd ~/rtd-rpi/update/

~/rtd-rpi/update$ ./update <id>

Replace <id> with the board stack address (0-7). Full instructions including recovery procedures are available at:

https://github.com/SequentMicrosystems/rtd-rpi/tree/master/update

DOWNLOADS

COMPLIANCE

RoHS and REACH Compliance Declaration

CE Certification

CE Test Report

WHAT'S INCLUDED

When you purchase the RTD Data Acquisition HAT you will receive the following items:

1. RTD Data Acquisition HAT for Raspberry Pi

2. Mounting Hardware
	Four M2.5×18mm male-female brass standoffs Four M2.5×5mm brass screws Four M2.5 brass nuts

3. Connector Plugs for all Inputs
	Nine 3-pin female mating connector plugs Two 2-pin female mating connector plugs

QUICK START

1. Plug your card on top of your Raspberry Pi and power up the system.
2. Enable I2C communication on Raspberry Pi using raspi-config.
3. Install the software from GitHub:
   ~$ git clone https://github.com/SequentMicrosystems/rtd-rpi.git

   ~$ cd /home/pi/rtd-rpi

   ~/rtd-rpi$ sudo make install

   ~/rtd-rpi$ rtd

The program will respond with a list of available commands. To read more about the CLI, please refer to the README file. If you would rather use a different platform, you can access the same functionality through dedicated, ready-to-use integrations in the Downloads tab above.