Asynchronous data access considerations
It is important to remember that many modern human-machine interfaces (HMIs)/supervisory control and data acquisition (SCADA) systems will access PLC data asynchronously. This means that instead of waiting till the end of the PLC scan to obtain or write data, the HMI/SCADA system will interrupt the scan to update or obtain data.
Normally, this is not an issue. However, when references via InOut interfaces are in use, it can cause unexpected behavior. This can be very difficult to diagnose because the next time data is updated, it may not be updated in the same place in the scan.
Have a look at the following screenshot:
Figure 8.13 – Example of an erroneous configuration with InOut interfaces
Figure 8.13 is an example of a configuration that is valid by the compiler but introduces issues if data blocks are interfaced with an HMI.
The blue data blocks represent asset data—they are connected to InOut...
