This document is relevant to: all Duet boards
Firmware versions: all firmware versions
The Duet series supports four types of temperature sensor: thermistor, PT1000, thermocouple, and PT100. This page describes the support for PT100 and other RTD (Resistance Temperature Detector) temperature sensors in RepRapFirmware for Duet electronics.
To connect a PT100 sensor to a Duet, you need an RTD interface board based on the MAX31865 chip. Duet3D produce a two-channel PT100 temperature sensor daughterboard for Duet 3 and Duet 2 mainboards.
Each daughterboard supports 2 channels, ie two temperature sensors per daughterboard. Duet mainboards and expansion boards support none, one or two daughterboards. Refer to the table below.
|Duet board||Number of boards supported||Pin names (RRF 3)||Channel (RRF 2)|
|Duet 3 Mainboard 6HC||2, stacked||spi.cs0 to spi.cs3||-|
|Duet 3 Mainboard 6XD||2, stacked||spi.cs1 to spi.cs4||-|
|Duet 3 Mini 5+||1||spi.cs1 to spi.cs2||-|
|Duet 3 Expansion 3HC||2, stacked||spi.cs0 to spi.cs3||-|
|Duet 3 Toolboard 1LC||0||-||-|
|Duet 3 Expansion 1XD||0||-||-|
|Duet 2 WiFi / Ethernet||2, stacked||spi.cs1 to spi.cs4||200 to 203|
|DueX2 / DueX5||2, stacked||spi.cs5 to spi.cs8||204 to 207|
|Duet 2 Maestro||1||spi.cs1 to spi.cs2||200 to 201|
Boards that support only one daughterboard can have either a Thermocouple or PT100 daughterboard connected, but not both.
There have been a couple of different versions of the PT100 temperature sensor daughterboard. The main difference is the configuration of the jumpers to select between 2-wire and 4-wire PT100 sensors. See the wiring section below for the function of these jumpers.
This image shows version 1.1, which has jumpers to select between 2 wire and 4 wire PT100 sensors.
Older board versions (pre v1.1) have solder pads to select between 2 wire and 4 wire PT100 sensors:
Each channel of the PT100 daughterboard has a 4-position terminal block. We'll number the terminals 1, 2, 3 and 4 in order (it doesn't matter which end you start from, because PT100 sensors don't care about polarity). Terminals 1 and 4 supply current to the sensor, and the voltage developed across the sensor is measured between the terminals 2 and 3.
The temperature daughterboard connects to the TEMP_DB connector on the mainboard. Make sure the temperature daughterboard is orientated correctly before connecting, see the wiring diagram for your mainboard.
We supply a plastic pillar with each daughterboard, to secure the daughterboard to the Duet or to the daughterboard below it. If the prongs of the pillar won't go into the hole in the circuit board easily, gently squeeze the prongs together with a pair of pliers first.
On mainboards that support more than one temperature daughterboard, the second daughterboard is connected by stacking it on top of the first. You can have two daughterboards of the same type (PT100 or Thermocouple), or have one of each.
This image shows a thermocouple daughterboard stacked on top of a PT100 board. The right-hand connector of the PT100 board has a 2-wire PT100 sensor connected to it. The left hand connector has a test resistor connected to it (see later).
On the first temperature daughterboard (ie closest to the mainboard), the terminal blocks labelled RTD1 and RTD2 will be the first and second temperature measurement channels respectively. If you stack two PT100 daughterboards, the terminal blocks labelled RTD1 and RTD2 on the upper board will be the third and fourth temperature measurement channels.
PT100 temperature sensors can be wired in either a 2-wire or 4-wire configuration. 3-wire PT100 sensors are also available, but these will need to be wired as a 2-wire, or an extra wire added to be wired in 4-wire.
2-wire connections have the advantage of wiring simplicity. 4-wire connections have the advantage of greater accuracy; the extra wires compensate for the resistance drop in the wires, making readings more accurate. 4-wire does not improve resistance to interference; better approaches to reduce noise include twisted wiring, isolation from noise sources, shielded wiring, better grounding of components etc.
See dc42's blog post here for additional information about using a 4-wire PT100 connection.
;Duet 3 6XD, Mini 5+ M308 S3 P"spi.cs1" Y"rtd-max31865" ; define temperature sensor number 3 as a PT100 on the first port of a temperature daughterboard plugged into the mainboard. ;Duet 3 6HC M308 S3 P"spi.cs0" Y"rtd-max31865" ; define temperature sensor number 3 as a PT100 on the first port of a temperature daughterboard plugged into the mainboard. ;Duet 3 3HC M308 S3 P"3.spi.cs0" Y"rtd-max31865" ; define temperature sensor number 3 as a PT100 on the first port of a temperature daughterboard plugged into the expansion board with CAN bus address 3. ;Duet 2 M308 S1 P"spi.cs1" Y"rtd-max31865" ; create sensor number 1 as a PT100 sensor in the first position on the Duet 2 daughterboard connector
To tell the firmware to use a RTD channel for one of the heaters, use the X parameter in the M305 command for that heater to specify the required channel (200 to 203 on the Duet 2 WiFi/Ethernet, 204 to 207 on the DueX5 or DueX2).
M305 P1 X200
This tells the firmware that for heater 1 (which is normally the first hot end heater) it should sense the temperature using the PT100 board whose CS pin is connected to NPCS0. The S, T, B, H and L parameters of the M305 command are not used. In firmware 1.20 and later, you can optionally use the R parameter to specify the value of the reference resistor if it is not 400 ohms.