just an FYI for everyone on the ZC21 two zone block from Xantech, it only has 1 mA of output on the common out, and even though the "distructions" (instructions) show the common out going into a 789-44 or compariable connecting block it will not work, I have tried, even talked with the techs as Xantech and they agree its a no go. Did not have much luck with their 795-20 either, but that unit has other issues.