Check your DS1820 pinout -  The part acts as you describe
when power & gnd are reversed. Note that the TO-39 pinout
in the data sheet is a TOP view.

Guess how I figured this out...  :)

Regards, Jack

Lieven wrote:
>
> I'm working on a PIC project that talks to a Dallas 1-wire part.
> Unfortunately, I can't seem to get it to talk to me.  Attaching
> a scope shows a presence pulse but no response to my (seemingly
> properly formed and timed, per my interpretation of the 1-wire
> spec) transmission.
>
> I'm surprised to say that neither the Dallas Semiconductor site,
> nor the Microchip site, nor anyplace else I've searched on the
> web, have any code samples for an iButton or 1-wire interface.
>
> Can anyone point me to a site, or send me an example?  C, assembly,
> even pseudocode would be handy so I could confirm that my bit-level
> signaling is correct.  My project uses the deluxe 17C7xx series,
> but I'm familiar with the lower end processors as well.
>
> Thanks,
> Lieven