Siemens SIMATIC S7-200 System Manual page 293

POSx_LDOFF Instruction
The POSx_LDOFF instruction (Load Reference Point
Offset) establishes a new zero position that is at a different
location from the reference point position.
Before executing this instruction, you must first determine
the position of the reference point. You must also move the
machine to the starting position. When the instruction sends
the LDOFF command, the Position module computes the
offset between the starting position (the current position)
and the reference point position. The Position module then
stores the computed offset to the RP_OFFSET parameter
and sets the current position to 0. This establishes the
starting position as the zero position.
In the event that the motor loses track of its position (such
as on loss of power or if the motor is repositioned
manually), you can use the POSx_RSEEK instruction to
re-establish the zero position automatically.
Turning on the EN bit enables the instruction. Ensure that
the EN bit stays on until the Done bit signals that the
execution of the instruction has completed.
Turning on the START parameter sends a LDOFF command to the Position module. For each
scan when the START parameter is on and the Position module is not currently busy, the
instruction sends a LDOFF command to the Position module. To ensure that only one command is
sent, use an edge detection element to pulse the START parameter on.
The Done parameter turns on when the module completes this instruction.
The Error parameter contains the result of this instruction. See Table 9-20 on for definitions of the
error codes.
Table 9-14 Parameters for the POSx_LDOFF Instruction
Inputs/Outputs
START
Done
Error
Open Loop Motion Control with the S7-200
Data Type Operands
BOOL I, Q, V, M, SM, S, T, C, L, Power Flow
BOOL I, Q, V, M, SM, S, T, C, L
BYTE IB, QB, VB, MB, SMB, SB, LB, AC, *VD, *AC, *LD
Chapter 9
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