Siemens SIMATIC S7-300 Manual page 206

Description of Functions
User DB allocation
Forward The program processes the block numbers in ascending order.
processing
Processing begins at Start, with the first block (specified
block number = 0).
If processing is to begin at some other point of the traversing program, spec-
ify the desired block number. Processing will take place by searching forward
to this block, then processing forward until the program end command is rec-
ognized.
Backward The program processes the block numbers in descending order.
processing
Processing begins at Start, with the last block (specified block number = 0).
If processing is to begin at some other point of the traversing program, spec-
ify the desired block number. Processing will take place by searching back to
this block, then processing in reverse until the program beginning is recog-
nized.
Note
If reverse processing is to execute the same sequence of movements as the
forward movement, the effects of the corresponding commands must be
taken into account in the programming. For example:
9-30
Data format
Byte 0
Byte 1
Byte 2 Direction of machining:0 = process forward
M outputs should be written separately in a block;
note M output (MD32) and G60/G64.
Note change between G60/G64 and G90/G91.
Note start and end of tool offsets.
M18 is not executed.
M02 and M30 at the end of the program are not processed.
Significance
Program number
Block number
1 = process in reverse
FM 354 Servo Drive Positioning Module