Analogue inputs and outputs
At all Domat modules, analogue inputs are separated from all the other parts of the module. However, a set of basic rules should be kept:
- resistance (passive) sensors should be connected directly to the module input terminals AIx, AIGND or AIC. In case of incorrect grounding, the voltage drop at the active input peripherials wiring may influence the passive temperature sensors. The sensors then show values up to 10 – 20 K higher than the real temperature.
- the same is for analogue output grounds (AOGND, AOC). A typical effect of this problem is that if a valve is opened, the measured temperature seems to be rising. At the commissioning phase, when the sensors are connected as the last part of the system, and valves are closed at the same time, this problem may not be identified.
The analogue inputs are resistant to voltage up to 24 V AC even if set as passive sensor inputs (R560, RCIO, RXIO). This does not apply if they are switched or jumpered as 4..20 mA (the internal 125 Ohm resistor may be damaged by the excessive heat caused by the flowing current).
Analogue grounds connection: left: correct, right: incorrect – the bold part of the conductor carries a voltage caused by the powering current of the active pressure sensor. The voltage brings a measuring error to the passive temperature sensor input.