ISP function is different for each type of AI module( SIL and Non-sIL) , and this is by design.
ISP control (Input Set as Predetermined) is used to avoid that unwanted values are used for input channels. A predetermined value is used for the input channels of the I/O module, if an input module becomes faulty or if the communication with the I/O system is lost.
When the controller detects a communication failure with an input module, the application variables are set to predetermined values. The reasons for a communication failure can, for example, be:
The cable between the I/O module and the controller is disconnected. The I/O module is not working properly due to hardware or a power supply failure.
As stated above for Non-SIL I/O AI810 ISP will never become active for underflow. But it will become active for channel error, module fault, or Controller to cluster communication fault,
There is 'underflow' state: 16#55 - this will not activate ISP,
There is 'very low level' state: 16#11 - this will activate channel error and ISP.
Wire break will generate 16#11 and will activate ISP.
This is the reason for AI880 working ok with on AI wire break.
For AI810, AI845 etc.
There is underflow state: 16#55 - this will not activate ISP,
There is no 'very low level' state at all.
Wire break will generate 16#55 but not ISP.
ISP is activated by taking out the module from its position.
The controller cannot know when it is open loop or the signal drops fast, therefore they have to treat this kind of problem in the application. R&D have made the Function block SignalInReal and the Control Module SignalInRealM to handle this. If the customer want the result "Keep current value" they shall set ErrorAtWarn = TRUE and use "Freeze" or if they want "Set ISP value" behavior they shall use "Predetermined". If "Freeze" is selected the FB/CM has save a four scan (application scan) old value and that will be used. Presumably that value was a good value when the signal suddenly has open loop. If "Predetermined" selected the signal shall set the value directly without dropping the signal.