Three combinations
There are three possible combinations for the alarm acknowledge bit (indicated with the * symbol).
No bit set
If no acknowledge bit is set, the alarm is intended to direct the operator's attention to a particular incident. He is not to take any special action (like stopping a motor or closing a valve). The alarm is internal in the PC and can be acknowledged by the operator.
Both To PC and From PC set
If an acknowledge bit is set both to and from the PC, this allows a "handshake" situation (provided that the PLC program can handle it). This implies the following:
When an alarm is triggered in the system and the operator acknowledges it, the acknowledgement will be sent to the PLC. The acknowledgement will then be returned to the PC (as an acceptance of the reception). Only then will the alarm be registered as acknowledged.
To PC set
An acknowledge bit to the PC makes it possible to acknowledge the alarm from the process, for example, by stopping a motor or opening a valve. The alarm can also be acknowledged from the PC. The acknowledgement is solely internal: the alarm will be marked as acknowledged in the Alarm List.