Siemens SINUMERIK 840D sl Programming Manual page 499

Coupling status
The coupling status of an axis can be checked in the part program with the system variable:
$AA_COUP_ACT[<axis>]
Value
0
8
Display of distance-to-go of the coupled-motion axis for modulo rotary axes
If the leading and coupled-motion axes are modulo rotary axes, traversing movements in the
leading axis from n * 360° with n = 1, 2, 3 ... , add up in the distance-to-go display of the
coupled-motion axis until the coupling is switched off.
Example: Program section with TRAILON and leading axis B and following axis C
Program code
TRAILON(C,B,1)
G0 B0
G91 B360
G91 B720
G91 B360
Job Planning
Programming Manual, 03/2013, 6FC5398-2BP40-3BA1
Meaning
No coupling active
Coupled motion active
11.1 Coupled motion (TRAILON, TRAILOF)
Comment
; Activate coupling
; Starting position
; Distance-to-go display at block start:
; B=360, C=360
; B=720, C=1080
; B=360, C=1440
Axis couplings
499