Hi Sara J,

when using C# for reading I/Os, I assume you are using S7 communication like is done by Snap7 (respectively Sharp7). In this case, the communication is asynchronous to the scan cycle.

Like other communications, it has effect on the scan cycle of the PLC. But depending on the amount of data, you might not recognize this.

Kind regards
Stefan

Stefan Arcularius

Hi Stefan,

I read 50 bytes of I/O of PLC from my C# application.

I couldn't see a noticeable change in PLC CPU scan cycle time.

I was curious to know if PLC postponing the C# commands and after some months of running can it cause scan cycle time increasing?

I'm also curious to know what is happening inside PLC, how PLC run C# requests, in which step of the running cycle?

 Hi Sara J,

Sara J

I couldn't see a noticeable change in PLC CPU scan cycle time.

With a modern PLC, you would have to exchange more data or do it in a very short period of time to achieve noticeable changes in scan cycle time.

Sara J

I was curious to know if PLC postponing the C# commands and after some months of running can it cause scan cycle time increasing?

Your C# commands are processed within the PLC by the PLC's operating system. The operating system processes the command as soon as possible - usually within milliseconds. When more commands are received, the commands are processed in an internal queue of limited size. A full queue causes the operating system to reject further commands. This does not result in a slowly increasing scan cycle time.

Sara J

I'm also curious to know what is happening inside PLC, how PLC run C# requests, in which step of the running cycle?

The read or write commands are handled in another process than the PLC commands. There is no synchronization between the PLC cycle and the processing of the commands.

This means when writing data, the data will be changed during on PLC cycle. This is different to the behavior of the process image, which is updated only before a new scan cycle is started. You should be aware of this, specially when the data you are accessing is used as IN_OUT parameter with call by copy. Also, you should never use this data inside the failsafe program because the asynchronous access might result in a "Data corruption in the safety program" error.

Kind regards
Stefan

Hi Stefan,

Thank you, your answer was great and helpful 

I use S7-300 CPU 315F-2 in this project.