S - S, Double Changing Arm, M4540 = 1 - HEIDENHAIN TNC 426 PB/M Technical Manual

S → S,
Double changing
arm, M4540 = 1
6 – 436
First the pocket number and tool number of the new tool are transferred.
7 7 7 7 Acknowledge with M4093.
Then the pocket tool number of the old tool and the tool number zero are
transferred.
Zero means clear the spindle!
HEIDENHAIN Technical Manual TNC 426, TNC 430