Direction Control with Dual Z Steppers

Setup: Smoothieboard 5XC v1.1, iOS, Octoprint, coreXY with dual Z steppers controlling bed height, PT100 controlled AC bed heater. Everything seems to be working right during homing and checkout. Z-axis min_endstop set to 0 with bed at hot end (up). Positive Z movement lowers bed. Z-axis homing brings bed back to zero. But starting a print z-axis tries to go up instead of down (lower). I could use a sanity check.


Imported from wikidot

What does M114 say at boot time, and what does it say when you are getting ready to print ? What does the beginning of your Gcode file look like ?

Send: M114
Recv: ok C: X:0.0000 Y:0.0000 Z:0.0000 E:0.0000

print ready:
Send: M114
Recv: ok C: X:0.0000 Y:0.0000 Z:0.0000 E:0.0000

start of Gcode:
;Filament used: 3.4151m
;Layer height: 0.2
;Generated with Cura_SteamEngine 3.4.0
M190 S60
M104 S200
M109 S200
M82 ;absolute extrusion mode
G28 ;Home
G1 Z15.0 F6000 ;Move the platform down 15mm
;Prime the extruder
G92 E0
G1 F200 E3
G92 E0
M83 ;relative extrusion mode
G1 F1500 E-6.5
G0 F3600 X42.211 Y20.23 Z0.3
G1 F1500 E6.5
G1 F1800 X42.451 Y19.784 E0.02527
G1 X42.835 Y19.299 E0.03086
G1 X43.186 Y18.935 E0.02523
G1 X43.565 Y18.6 E0.02524
G1 X43.992 Y18.328 E0.02526
G1 X44.641 Y18.071 E0.03483

could this be the culprit?
G1 Z15.0 F6000 ;Move the platform down 15mm

What happeens if instead of starting the gcode file, when it’s all ready/homed, you just send G1 Z15 ?

Are you *sure* you have your axis the right way around ? Maybe try inverting it ( will also require inverting homing direction ) and see if it works then ( in which case you were just confused about how the axes are supposed to be setup for your machine ). A video might help.

The G1 Z15 after homing worked as expected. I changed the starting Gcode as follows and all worked well.
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
M107 ;start with the fan off
G28 Z0 ;home Z
G1 Z15.0 F300
G92 E0 ;zero the extruded length
G1 F200 E3 ;extrude 3mm of feed stock
G92 E0 ;zero the extruded length again
G1 F840

Thanks for your help. Your suggestions jogged some new thoughts into the old cerebellum.