Modbus RTU Accepted No Response / Timeouts Percentage

[69FIREBIRD]

Hello, I want to find some documentation/standard on accepted percentage of no responses/timeouts. i.e. is 1/100 1/1,000 1/10,000 1/100,000. I had a quick skim through modbus.org documents but nothing stood out.

I have a network where there is an issue with proprietary Lighting Controllers not responding to commands from my 1756 Prosoft Master. I know why its not responding (-11 response code) and this is a bigger issue related to the Lighting Controller hardware/firmware and passing on commands "downstream" to the light fitting installation over powerlines.

We are able to insure a command does gets through by tuning up timeout time, time between commands and retries and because we're dealing with gradual lighting changes its not a big issue.

I have a colleague who says 1 timeout/no response in a year is not acceptable in some industries and in his case this generates an alarm condition. Personally I think this is excessive, so finding some documentation that defines acceptable percentages for my recommendations would be great.

Thanks for the feedback in advance.

 

[Epy]

Your colleague is smoking crack. Some instruments will have no misses but others are almost guaranteed to miss messages. I shoot for <1% but sometimes have to accept as high as 3-4%. Some instrument manufacturers just suck at implementation and it can't be helped.

 

[Saffa]

I still have some old MB+ networks. If there are more than 2 retries in a month I go looking for a fault.

But it's horses for courses really. If you were using modbus over radio then you'd tolerate 98% success rate in a day.

So the answer really is... it depends.

Will the dropped messages affect operation? If not, then establish an acceptable base line and at up an alarm if that is exceeded by more than say 5%.

Then look at resolving why you're getting an error by talking to the equipment supplier to sort their issue out.

 

[Peter Nachtwey]

MB+ and MB rtu are two different things.
I do not like MB rtu because few people implement it correctly so there are compatibility issues.
It all boils down to measuring the time between characters.
http://stackoverflow.com/questions/20740012/calculating-modbus-rtu-3-5-character-time
I hated when I got the tech support call due to someone else's sloppy implementation of the timing specifications.