# Backward movement with external position offsets

## External position offset

During “forward/backward on the path” the programmed path may be offset by external online influences.

This position offset can be signalled to the entire NC channel (see #CHANNEL INIT[CMDPOS]) by subsequently synchronising the channel. This deletes the offset, i.e. an absolute programmed position then does not include an offset.

## Backward motion by position initialisation with offsets

In backward/forward motion, external offsets are not moved back in the same way as other motions specified in the NC program. If a backward motion moves to a position initialisation with an external offset, there are 2 options. They can be set using the channel parameter P-CHAN-00275:

1. P-CHAN-00275 is not set (default); no further backward motion is permitted since the positions specified in the NC program must also be approached without offset, even in a backward direction.
2. P-CHAN-00275 is set; the offset is retained and the backward motion may resume. The current absolute positions defined in NC program are no longer valid since they are shifted by the currently valid offset.

## Notice

If P-CHAN-00275 is set, backward motion reverts to an offset path starting at the position initialisation. See the figure below

Possible offsets are:

- Manual mode actions
- Offsetting path by 'Jog of Path' (see [FCT-C15] “Insert a program“)
- Values are added to the position controller via the additional interface
- Axis coupling via PLC
- Corrections via the online tool compensation

Motion in backward/forward direction with offsets
Program execution with offset overlapping

## Attention

An active kinematic transformation which is especially dynamically different depending on the position, may cause a dynamic overload (or an incomplete loading) of the axes.

Background: If a backward motion has an offset, a deviation is made from the original path. However, dynamic planning was executed with the original path motion in forward direction without offset.

## Notice

Online influences during backward motion such as

- Measurement with G100,

- Homing with G74 or

- Deselecting online tool compensation by #OTC OFF if P-CHAN-00275 when P-CHAN-00275 is not set

The backward motion is ended and the warning is output.