I can home but not run GCode

LKasdorf

I have my homebrew CNC basically working, but I seem to have a setting issue. I am using a DDCS V3.1 controller. I have physical home switches and have set up software limits. My XYZ limit switches are NC and I'm currently connected to HomeX, HomeY, and HomeZ inputs. Not the X-- or X++ inputs, etc. I have set up soft limits on all axes. This approach was suggested by somebody, and seems valid- at least homing works.
I can home all 3 axes just fine. The problem is when I go to run even the simplest GCode.
I am running code like this:
G21 G17 G90
G0 X100 Y100
..simply in an attempt to get X and Y to move. What happens is that for some reason, it starts moving the Z axis up (past the homing point of 0 and into positive values). Quickly, it throws a "Z axis pos soft limit" error. I didn't ask it to do anything with Z! Even if I explicitly tell Z to go to a valid position, like this, it does the same:
G0 X100 Y100 Z-15
I wonder if there is some setting that I am missing. Here are the values of some that might be relevant.
soft limit Z-  -110
soft limit Z+  0
(I don't understand the following)
home signal effective when home: continue HOME
enable limit signal when home: enable
enable Z home searching: enable
Would it be better to wire my switches to the X--, Y--, Z-- (or should I use Z++?) inputs rather than the home inputs? My Z axis goes from 0 to -120mm. If I use these inputs, what settings would I use for these to serve as Home also?
I open to any ideas or suggestions!