In the manuals i did not found a description of how the subsciption and reading rate from a device is calculated. It would be good to have a bit more information to tune the scan class rates.
A little background:
I use a protocol converter to connect old Siemens S5 PLC’s to Ignition using the S7 driver. Since the connection between the converter and the PLC is a 9k6 serial line, i have to keep an eye on the update rates.
In Ignition i have two different Scan Classes (2s and 15s) that read 2 different memory blocks. The 2 second ScanClass is behaving as expected and results in a read request every 2 seconds.
For the 15s Scan Class i see in the Server Diagnostics a subscription rate of 5s. The i switched the read request to trace and found that the two reads are combined to a single request executed at 2s.
I don’t think that there is an easy way to change this behaviour, so i will have to change the plc program to combine all tags in one memory block and put everything in a single ScanClass.
Can you please give me some more information about the internal workings of the OPC drivers:
- When is the subscription increased compared to the scan class rate
- Under which conditions are memory block reads combined in the S7 driver.
Thank you for your help, regards