Thank You for the fast answer.
Yes, it's TTL Output from the Sensor (SHT75).
They also use 8-bit CRC which i read somewhere is not good to handle with the PLC?. We also looked at the Dallas 1820 1-Wire Temperature sensor/transmitter but they also use crc causing more complex calculations.
Using the ULN2803A IC and NPN Input sounds very symphatic to us. Have you a sample how to capture/process this style of input (like sample for Analog I/O in the Manual)?
We have already seen possibility to use 4-20mA with 250 Resistor to get 1-5V. This would be used for some Sensors. Unfortunately, Temperature and Humdidity is not covered without expensive transmitters. We have found something interesting @ Airtesttechnologies.com, supplying 4-20mA for Temp, RH and CO2. We contact them.
Thanks for the fast and accurate answer, very good service!