When resistance thermometers (Pt100, Pt1000, Ni1000) are used, a constant current must flow through them to build the necessary voltage drop for the evaluation. For this, the I/O module provides a constant current source which is multiplexed over the max. 8 (depending on the configuration) analog channels.
- 1
-
Return line
- 2
-
Twisted pair within the cable
If several measuring points are adjacent to each other, only one return line is necessary. This saves wiring costs.
With the 3-wire configuration, two adjacent analog channels belong together (e.g. the channels 0 and 1). In this case, both channels are configured according to the desired operating mode. The lower address must be the even address (channel 0), the next higher address must be the odd address (channel 1).
The constant current of one channel flows through the resistance thermometer. The constant current of the other channel flows through one of the cores. The module calculates the measured value from the two voltage drops and stores it under the input with the higher channel number (e.g. C1).
In order to keep measuring errors as small as possible, it is necessary to have all the involved conductors in the same cable. All the conductors must have the same cross section.
|
Pt100 |
-50 °C ... +70 °C |
3-wire configuration, two channels used |
|
Pt100 |
-50 °C ... +400 °C |
3-wire configuration, two channels used |
|
Pt1000 |
-50 °C ... +400 °C |
3-wire configuration, two channels used |
|
Ni1000 |
-50 °C ... +150 °C |
3-wire configuration, two channels used |
The I/O module performs a linearization of the resistance characteristic.
In order to avoid error messages from unused analog input channels, it is useful to configure them as "unused".