Siemens simatic et 200s Operating Instructions Manual page 211

Rotary axis
You determine the direction in which the destination is approached by selecting the
directional enable (DIR_M, DIR_P):
Control bits DIR_P and DIR_M
DIR_P = 1
DIR_M = 0
DIR_P = 0
DIR_M = 1
DIR_P = 1
DIR_M = 1
Figure 5-10
ET 200S Positioning
Operating Instructions, 05/2007, A5E00124871-04
Execution of Absolute Positioning
5.6 Functions of the 1PosSSI/Digital
Direction
The destination is approached in the plus direction.
The destination is approached in the minus direction.
The destination is approached by the shortest route. The
1PosSSI/Digital works out the direction in which the
destination is approached.
1PosSSI/Digital
211