RTD Data Acquisition 8-Layer Stackable Card for Raspberry Pi

The RTD Data Acquisition Card offers a compact and inexpensive solution for reading and storing data from up to 64 RTD-100 temperature sensors with a Raspberry Pi.

Using two ADS1248 24-bit delta-sigma converters with four channels each, the card achieves better than 0.1%. Field calibration with a precision 100Ω resistor can lead to 0.01% precision.

Since temperature measurement using RTD is based on resistance, the resistance in the lead wires and connectors must be taken into account when calculating the overall resistance in the system circuit. Using a 3-wire circuit design allows lead wire resistance to be factored out of the overall calculation.

Features

• Two ADS1248 24-bit delta-sigma converters (four channels each)
• Factory accuracy: 0.1%
• Maximum accuracy (through calibration): 0.01%
• Eight layer stackable to 64 RTD channels
• RS485/MODBUS transceiver
• PT100/PT1000 sensor selection
• Programmable threshold LEDs on all inputs
• Pluggable Connectors 30-16 AWG wires
• On-board hardware watchdog
• Reverse polarity power supply protection
• On-board resettable fuse
• Command line
• Python library
• Node-Red nodes
• Modbus RTU
• Open Source Hardware Schematic
• 3D Printing Enclosure

Feature Details

Compatability

The card is compatible with all 40-pin Raspberry Pi versions from Zero to 4. It shares the I2C bus using only two of the Raspberry Pi’s GPIO pins to manage all eight cards.

This feature leaves the remaining 24 GPIOs available for the user.

Power Requirements

The RTD Data Acquisition card needs 5V to operate and can be powered from Raspberry Pi. The cards require 50mA to operate.

Stacking Multiple Cards

Up to eight RTD Data Acquisition cards can be stacked on your Raspberry Pi.

Each card is identified by jumpers you install to indicate the level in the stack. Cards can be installed in any order. For your convenience, two stack jumpers are provided with each card.

DIN-Rail Mounting

The RTD Data Acquisition card can be installed parallel on a DIN-Rail using the DIN-Rail Kit Type 1, or perpendicular using the DIN-Rail Kit Type 2.

Software

Software Interfaces

You can write your own Data Acquisition system in C, C++, PERL or the language of your choice using the Command Line functions or the Python Library. A browser interface can easily be deployed using Node-Red.

Field Calibration

Field calibration to 0.01% precision can be achieved using an external high precision resistor. Software commands permit the user to calibrate the card in two points (the 2 points must be as far as possible but inside the measurement range).

The resulting values are stored in flash and used for resistance measurements compensation witch will reflect in the temperature readings.

The resistance to temperature conversion is made by the simplest equation T = (R - R0)/(R0*k), where T is temperature in Celsius degree; R is the sensor resistance; R0 is the resistance at 0 deg Celsius (100 for PT100, 1000 for PT100) and k= 0.00385. If the user needs to use the polynomial equation, the resistance measurements are available.

RS485/MODBUS Communication

The card contains a standard RS485 transceiver which can be accessed by the local processor or by Raspberry Pi. The desired configuration is set from two bypass jumpers on configuration connector J3.

See in the CARD LAYOUT SECTION jumpers "485-RX" and "485-TX". If jumpers are installed, Raspberry Pi can communicate with any device with an RS485 interface. In this configuration, the card is a passive bridge that implements only the hardware levels required by the RS485 protocol. To use this configuration, you need to tell the local processor to release control of the RS485 bus:

~$ rtd [0] rs485wr 0 0 0 0 0

If jumpers are removed, the card operates as MODBUS slave and implements the MODBUS RTU protocol. Any MODBUS master can access all the card's inputs, and set all the outputs using standard MODBUS commands. A detailed list of commands implemented and parameters addresses can be found on GitHub.

In both configurations, the local processor needs to be programmed to release (jumpers installed) or control (jumpers removed) the RS485 signals. See the command line online help for further information.

LED Threshold Signals

The RTD board has eight LEDs that can be activated when the input reaches a preset threshold. The threshold can be set in software for each input.

PT100/PT1000 Sensor Selection

Each group of four channels can be preset to read PT100 or PT1000 sensors. Selection is made from the bridged pads placed on the left or right side of the board.

The board is shipped with bridged pads instead of a the jumper selector (PT100). The calibration resistors used onboard are 0.01%. We did not install a mechanical jumper because the resistance of the jumper could influence the precision of the measurement. If you want to use the board with PT1000 sensors, please remove the bridge between the jumper pins.

Firmware Update

The card firmware can be updated in the field by running a command. The update is made with the latest firmware version located on our servers. More instructions about the process can be found on GitHub.

Please make sure there is no process, like Node-Red or python scripts, that tries to access the card during the update process.

Card Layout

Note: the PT100/1000 2-pin jumpers have been removed in place of solder bridging due to the possible resistance of jumpers being too great for the 24-bit ADC. Milliohms matter!

Package Contents

• 1x RTD Data Acquisition Card
• 4x M2.5x18mm male-female brass standoffs
• 4x M2.5x5mm brass screws
• 4x M2.5 brass nuts
• 2x Pin Jumpers
• 8x 3-pin mating connector plugs
• 2x 2-pin mating connector plugs

Mechanical Specifications