Profinet Update Rate

PLCnext Technology & PLCnext Controls
1

Hi,
We currently have two profinet AXL F BK PN (profinet) bus couplers connected to a 3152. These bus couplers have a handful of Axioline IO on each. At present, we have not configured the update tasks for profinet in plcnext engineer. Am I correct in my understanding that right now the IO data from the bus couplers would only be updated to the PLC at the current default rate of 50ms (20Hz) ?
Similarly, with IO data that is coming from the axioline bus connect the 3152, is the rate at which this is updated and available set by the ESM system variables update task in PLCNext engineer (again default to 50ms). This is also a bit unclear to me.
Cheers,
Lindsay

2

Hi Lindsay,
for the Axioline bus you can set-up a trigger task. If this is not set, the update time is calculated by the firmware.

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For Profinet, the task settings are only used to update the PNIO_* and PND__S__ system variables. If no task is selected, then these variables are updated every 50ms. The update time needs to be set-up for each device individually in the interface section of each device.

3

Hi Stefan, thanks for the prompt response. A few follow up questions :
Trigger Task :
* Is this a new feature ? I don’t seem to have this option in PLCNextEngineer 2023.0.1 . My understanding from the hint above is basically that if not set, firmware will just automatically handle and basically at the start of execution of every cyclic task the requisite IO will be updated with the latest data.
* Profinet update rate : Since we are using a Profinet bus coupler, I don’t explicitly deal with the PNIO_* or the PND S_ system variables (rather we just scan and then add IO from the bus coupler to our project and away we go) . In the background there is some cyclic exchange over profinet with the bus coupler, I am just not sure if it uses any of the above variables to achieve this exchange. My question remains the same, If I have a profinet bus coupler with IO, how frequently will the GDS/RunTime get new IO data ? The motive here is that if I have a task on the Plcnext that is executing at 1KHz or so, looking for a rising/falling edge on some digital IN that is connected to the bus coupler, yet if IO data is only updating at 20Hz or what not because of the profinet comms cyclic rate, then I will not be detecting edges at 1KHz, rather at 20Hz and thus have a time error closer to 50ms as opposed to 1ms.
Cheers,
Lindsay

4

Hi Lindsay,

the oldest version of PLCnext Engineer that I have running is 2023.0.5. There the Trigger Task is available in the Axioline settings. The Profinet IO data is written to the GDS with the individual cycle time (reduction ratio) of each Profinet device. By default this is set to 8 ms.

5

Stefan,
Thanks for your prompt response. This all makes sense.
Cheers,
Lindsay

6

Stefan,
Sorry one more question. By default this is set to 8ms. How do we set it to something smaller ? (1ms). I recall reading somewhere else that this is dependant on the number of profinet devices which default to 128. If we set the number of profinet devices in PLCNext Engineer to 32 will we have a 1ms cycle time ?
Cheers,
Lindsay

7

Hi Lindsay,
to adjust the cycletime you can double-click on the interface part of the device.
image.pngAnother way is to double-click on Profinet in the plant section and adjust the times in the “Interface List”.
The smallest possible cycletime depends on the device itself, the controller, the total number of devices in the network and the line depth. Here is a recommendation from the PO as a rule of thumb. Each device has a “Minimum Device Interval” in its GSDML file.
image.pngThe line depth is the number of switches between the controller and the device.

8

Stefan,
Perfect. This all makes sense and is clear.
Cheers,
Lindsay