Synchronize MII transaction - only one running instance at a time

Question

Hello,
I have a scenario where I use an MII transaction triggered by PCO, and it may be the case that the same PCO notification triggers this transaction twice within a very short time, i.e., the second transaction instance starts its execution before the first instance has finished. But I really need to make sure that those calls are done one after the other and that they do not overlap.
What is the best way to achieve this? Which MII technologies can I use? I do not want to use message services and schedule a job that handels the requests one after the other - I need real time action and  in this case I would need to schedule the Job every second. Im really looking for a kind of semaphore/mutex solution to make sure that the MII transaction call can be fired by PCO and in case the previous transaction call is still running, the current one should wait until the previous one has finished.
Thank you for your input and ideas!
Best Regards,
Matthias

Answer 1

Hi Steve,
1. The PCO notification will be triggered when a PLC counter increases (which is actually the trigger that another piece has been produced on one of our production lines) - this happens in average every 10 to 15 seconds, but due to some special setup it can be the case that 2 or 3 consecutive pieces come out at the end of the line immediately one after the other, followed by a larger pause.
2. The expected duration of the MII transactoin will be approx. 2 to 3 seconds.
3. The exact software versions we use are PCO 2.1.6.1 and MII 12.1.9.123.
to be precise, the serialization of the transactiona does not need to be for all instances, but only for instances of the same production line, i.e., transactions of different lines may run at the same time, but for a single production line the transaction calls must not overlap.
The reason is the following: We want to implement a "difference posting" of produced pieces: each PCO message holds the  PLC counter value, and in the MII transaction I want to refer to the counter of the previously produced piece. If the difference is one, everything was OK, one additional piece needs to be recorded in the system. If the difference is greater than one (for the sake of example, lets say 3) I will record 3 new pieces in the system, and if the difference is less than one (wrong order in message delivery, should normally not happen) I will simply ignore the message. This gives us the possibility to not lose data during a PCO outage: PCO is up until PLC counter value 10, is down from PLC counter value 11 to 15, and comes back at PLC counter value 16. When I process the message with counter value 16, I will see that the last processed piece was for counter value 10, therefore I need to create 6 pieces - the 5 I have "lost" plus the current one.
To be able to safely refer to the previously produced piece, there must not be any parallel transaction executions, since then maybe two instances read the same "previous counter" and therefore mess up the collected data.
I hope this gives you an idea what I want to achieve.
Best Regards,
Matthias