Immediately write outputs - more on modbus plus performance

more on modbus plus performance
I have more dilemmas considering modbus plus. I'm trying to reduce the unnecessary traffic as much as possible, so I've unchecked "immediately write outputs" in advanced settings for modbus plus hoping that lookout will write to plc only when modbus object is polled. However, it seems that it doesn't happen that way, looks to me like lookout is writing to plc poll or no poll. I need to cut down the traffic because I have delay in system response and I'm considering lookout's share in it.
I have a feeling that making a system with deterministic response (which should be one of mb+ highlights) is quite cumbersome and sometimes impossible.
Can anyone comment on this issue and maybe share ideas for improving modbus response time? Is there anything about modbus plus and lookout on ni.com that I'm not aware of?
Darko

By default, Lookout writes Outputs immediately (without waiting for the poll). BUT, if you uncheck the "immediately write outputs" setting, it should write only when polled. If it's not doing this, it's a bug -- can someone at NI verify this?
Couple of things you can try for improving communication performance:
1. Make sure you're not writing outputs every-so-many polls (unless you really have to). By default, Modbus Object writes the Outputs every 100th poll -- even if none of them have changed. Use the Modbus INI settings to change this behaviour:
http://digital.ni.com/public.nsf/allkb/2E64D5CF87CA6A1086256BB30070DC1A
2. If you have some IO points which don't need to be read as fast as others, use a second Modbus object for these with a slower poll rate (but on the same address, etc.).
3. Use an appropriate update deadband for your analog reads -- you can filter out "noise" from data this way. Note that the IO are still read every poll, but aren't propagated to the rest of the system unless the deadband is surpassed.
Hope this helps,
-Khalid

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Lookout modbus plus performance

I'm running 9 Schneider Electric PLCs Compact 265 connected to Lookout v5 PC via modbus plus. I have cyclic stalls of transport, each 50 secs or so, which last for 3 or even more seconds. I've tried all the settings there are on modbus object in Lookout, but with no success, only different graph shapes in performance monitor (you can have a look at graphs at the end of mail). Even single PLC on network (with others shut down) freezes the communication for more then second, which is unacceptable since I need to refresh my data at least each 350ms.
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I've checked scan time in PLCs, they're all running smooth with scan time under 90ms. This time doesn't increase even during modbus peaks, I've reduced watchdog to 100ms and it never triggered. Looks to me like plc has more than enough time for modbus, so plc should not be the problem.
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3. modbus underperforms
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PLC uses 130 registers of memory (type 4 – holding register). With all registers back to back (no slack between registers), this should mean that all the memory can be transferred in 2 frames. This also means total data load per sec should be
9 plcs * 3 refreshes * 130 reg * 2 bytes = 7030 bytes.
Even with overhead included, this is so little compared to theoretical 120 kbytes modbus plus should be able to perform, so nothing else for me to do but scratch my head.
I've checked Lookout source for any loops which could initiate some read or write at 50sec cycle, but there's nothing even close anywhere in program.
Am I missing something? Can anyone tell me why modbus freezes my system?
Bye everyone,
Darko
SVETLOST TEATAR

Hello,
It’s been a while since I last checked the forum, but in the meanwhile I’ve resolve the problem.
The solution lies in modbus.ini and the way modbus object communicates with modbus secondary, an issue not properly covered by help and ni.com. Unless defined otherwise in modbus.ini, modbus object will refresh all of the configured memory points in your plc each 50 or so polls (I can’t remember which is the default value). This can be a problem if the number of memory points is large, because it congests modbus and creates performance stalls.
You can read more about this here: Lookout Driver Objects - Custom INI File Settings (http://zone.ni.com/devzone/conceptd.nsf/webmain/E54F75254BBE772786256CFC00700B5F).
Thanks everybody for joining in.
Bye,
Darko

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Solved!
Go to Solution.
Attachments:
Force negative value I16.jpg ‏48 KB
Force negative value I32.jpg ‏48 KB

marcello.zoppetti wrote:
How can I track the CAR?
Just watch for it in the new release notes for a version of LabVIEW and the patches. NI does not make their bug reports public.
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Immediately write outputs - more on modbus plus performance

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