DS1990 not working

I have some Dallas keys laying around, but from day one I have been able to neither read nor emulate them. Today I tried the new iButton fuzzer app and checked the signals with an oscilloscope, probing pin 17. The Metakom and Cyfral emulation seems to be working fine, with the signals going through fine. As for DS1990, pin 17 seems to stay high no matter what. Could this somehow be a hardware issue with my particular Flipper?