1978   How to Cancel an operation in excecution.

Created: 16 May 2025

Status: Discussion (red)

Part: Part 7-2 (2020; Edition 2.1)

Links:

Page: Table 108, Fig 43, Fig 44

Clause: 20

Paragraph: 20.1, 20.3

Issue

Table 108 defines the generic behaviour:
A Cancel_req received during a WaitForChange state results in a Cancel_resp-(command-already-in-excecution).
This behavior is understandable for SPC, DPC, ENC, but does not allow to interrupt/abort an operation of an APC, INC, ISC.
APC, INC and ISC, depending on the process it is attached to, may need time to reach the requested position, and a change of operation may be needed prior the expiration of the operTimeout (monitoring time during whch the change is to be expected).

Proposal

Propose to differentiate the generic behavior and the state machine, so that a cancel_req may be sucessfully excecuted during the state WaitForChange.

Discussion Created Status
change to discussion 17 Jun 25 Discussion (red)
The intend is to cancel the control state machine to reset the control state machine, not to halt a control movement.
The same occur when time out occurs. The state machine is reset but it does not necessarily halt a control movement.
Propose to allow the usage of cancel during the "waitForChange" state.
The controlled object may respond with a
- CancelResp- (command-already-in-excecution) in case it does not allow the cancel to happen,
- CancelResp+ in case it does allow the cancel to happen - in that case the state machine transitions to Ready (direct control model, select control model operate-many), unselected (select control model operate-once).

The purpose to allow Cancel during the WaitForChange state is to reduce the waiting for the timeout condition. 		17 Jun 25 Accepted
If the control is canceled, then the equipment has no active command anymore.
An APC can issue command to the external equipment using higher and lower binary output. APC does not mean that the electrical connection to the process is a analogue value.
Canceling the operation would mean that the APC does not issue higher or lower command to the process, and the process would stay in the state it is.
05 Jun 25 Triage
I disagree that this functionality is needed or even ever desired.

For example, a tap change command cancelled partially through the change encompassing multiple taps. Upon cancelation then what does taps changer do? Return to original position or remains at some random position between original and commanded tap?

I can imagine many IOP issues that will result if this proposal is accepted 		16 May 25 Triage

 

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