I realized that I needed to keep track of the various voltages I have available on my layout so I created this visualization.
It is just a visualization, I have no idea how the digital signal frequency and amplitude compare to the 16VAC supply. I don't recall how negative the DC current is that comes from the Viessmann signal modules. It does mean that next time I have to do something I can come back to this and know what is positive and negative relative to the common ground.
As it turns out, the 5 Volt supply combined with the resistor already in the signal circuit was too dim (since they selected the resistor for 16V AC) so I simply connected the wire to the nearest 16VAC supply bus after all instead of replacing the resistor with a smaller one.
Update 2007-08-12I added the outputs of a Märklin 6080 loco decoder. The function output is about -11 Volts relative to ground. With a capacitor this increases to -14.4V indicating that it is not a clean DC output. The motor outputs are never off and my meter measures between -5 and -15V of the pulsed width output. A capacitor locks this at -17 Volts.
6080 decoder wire colors: Yellow = f0 reverse, Gray = f0 forwards. Green = motor forwards, Blue = motor backwards, Black = motor return, Red= digital, Brown = ground.