Ugh, I don't know how I missed this. When making up the cables for the DACs I realized something; for the DACs I need +5 (on)and 0(off), and I can get this with the FB's switched-low digital outputs and a pull-up. BUT... In order to get +5 to the DAC, the output needs to be off (no current through the pull-up resistor connected to +5V), with the Digital Output on, full corrent flows and the output drops to 0V. This works, I think, but it's not ideal. With +5V to each DAC's input, the DAC's output becomes maximum (4.2V) which, through the opamp and feedback circuit, creates the maximum output. I did not invert things. What this means is, the default or failed position of the interface's outputs which replace the dials is FAN -100%, Max Heat, Defrost. Not the most ideal failure mode, especially in LV. But, if the outputs fail, then the relay drops out, removing PC control. Which means Fan-Minimum, Heat (wherever it's set), Mode- Max AC. Ok not so bad, but I just don't like the default-on outputs here.
I wonder if sending the DAC output to the inverting terminal of the opamps instead of the non inverting would make me happier. At a minimum, I have to add another small board with another 17 pull-up resistors to +5V, and more
headers. I have a little room left. I have to be kind of careful here, I don't have a lot of room between the halves for cables/wires. I'm probably going to have to make the box a little taller (good thing it's made out of ABS).
I'm open to suggestion.
