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I faced a new parameter for synchronization in CANopen: Synchronous Counter Overflow.

Synchronous Counter Overflow
The synchronous counter overflow object is an object dictionary entry that defines the maximum value of the SYNC counter. 
The SYNC counter is an optional part of an SYNC message. 

You can enable the SYNC counter by specifying a non-zero value to the synchronous counter overflow object.

The SYNC counter value starts from 1 and increases by 1 with each SYNC message. 
An SYNC cycle is the time period between the time when the SYNC counter value is 1 and the time when the SYNC counter value reaches the synchronous counter overflow value. 
At the end of an SYNC cycle, the device sends out the SYNC message with a counter value that equals the synchronous counter overflow value. 
The device then resets the SYNC counter value to 1 for the next SYNC message. 
The next SYNC message starts another SYNC cycle.

Use the synchronous counter overflow object to ensure periodic SYNC events occur in SYNC cycles with the same SYNC counter value. 
You can use specific SYNC counter values to trigger multiple periodic SYNC events.
For example, you can set the periods of event A and event B to 3 and the period of event C to 4. You then set the synchronous counter overflow to 12.
When you execute the application, event A and event B occur when the SYNC counter is 3, 6, 9, and 12. Event C occurs when the SYNC counter is 4, 8, and 12.

https://www.ni.com/docs/en-US/bundle/ni-industrial-communications-canopen/page/canopenhelp/canopen_sync_object.html

I know that we can set this object in command 0x1019 to count SYNC messages, and of course, a maximum is set for it, but I don't understand why this is used.

Sajadi
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1 Answers1

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It's just a counter with configurable overflow. Devices can use it for whatever, so check the docu of your devices (or pick your use-case if you are writing your own). For many applications you don't even need sync at all.

According to [1] it is commonly used to distribute bus load. You certainly can do a lot of applications without it, simply doing everything at the same period. It's easy to calculate the bus load in this case to avoid overload. Especially because the COB-IDs also imply strict priorities, so you can have "background traffic" that doesn't invalidate your calculations for the high-priority messages.

[1] https://www.can-cia.org/can-knowledge/canopen-fd/sync-protocol/

maxy
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