Task Priority Documentation?

2025-08-11T08:59:00.607000Z

Hi All

I was watching the Wago PFC300 webinar ( https://www.wago.com/global/lp-thank-you-onlineseminar-pfc-300 ), and the presenters comment about priority 1 being way above the Ethernet task priority, so that a priority 1 PLC program can actually loose SCADA connection, made me try to round up some hard evidence on Codesys Priorities.

I find there is very little information on "correct" task priority configuration. There is a lot of different hardware manufacturers, using Codesys Runtime, that has different descriptions in their manuals on how to set up priority.

https://doku.amk-motion.com/en/Subsystems/PDK_202975_A-Serie_en/Content/Projekt/AMKAMAC/PDK_202975_A4-A5_Produktbeschreibung/Funktionalitaet/PLC-Programming/PLC%20Tasks.htm?TocPath=PLC-Programming%7C_____7

https://product-help.schneider-electric.com/Machine%20Expert/V1.1/en/SoMProg/index.htm?#t=SoMProg%2FTask_Editor%2FTask_Editor-7.htm&rhsearch=priority&rhhlterm=priority&rhsyns=%20&SUBSTRSRCH=1

https://content.helpme-codesys.com/en/CODESYS%20Development%20System/_cds_f_task_configuration.html

So my questions :)

1) If there is only 1 PLC_PRG task, does priority matter?
2) What is the priority of Ethernet / communication, if nothing is entered?
3) What is the priority of Codesys OPC UA server?
4) Some users here on forge mentions that Wago PLC programs should run as priority 11, but where does this recommendation origin from?
5) What are you recommendations for a relatively slow process, like building automation? It uses OPC UA for communication to SCADA, maybe some MODBUS-RTU and MODBUS-TCP, usually around 100 I/O, been running with everything in a single task at priority 1, 50-100ms cycle time. Some controllers have separate MODBUS tasks, but doesn't seem like a big difference. How would you recommend splitting up communication, IO fieldbus and programs into differently prioritized tasks?

I know its a complex matter and all about my application, but I really miss there was just some kind of guidance in the documentations and with all new standard Codesys programs having priority 1 as default, makes it seem less important, than it really is.

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Task Priority Documentation?