Mitsubishi Q on profibus


We are rebuilding a plant where some MCCs were destroyed in a fire. In order to get back into production asap we decided to go profibus (simocode) on all the drives. This necessitated a switch from the oldish A3A CPU to the latest Q25 CPU ( 7 PLCs total).

What concerns me is that the Mitsubishi PLC profibus cards (Q71PB92D) can only handle 30 nodes "due to card memory". I am accustomed to 32 nodes per profibus DP master and based on the fact that we are installing 76 optical link modules (to give you an idea of the network size) - should I be concerned with the ability of this Japanese PLC to handle Profibus? Any field experience out there?


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The 32 node limit is a side effect of the RS485 that profibus runs on, it shouldn't be a limitation of the controller or scanner. In theory you could have 126 nodes on one bus with several repeaters (or OLMs) thrown into the mix. In practice you will find that not all vendors have slave node addressing beyond 99 and some profibus masters limit the amount of data from each slave to a fraction of what the standard allows for.

I suspect that Mitsubishi allowed for the maximum 32 nodes per segment, one master, one configuration node and 30 slaves.

In general you will find extremely good support for Profibus from european vendors and good support from north american vendors, with the exception of rockwell (but very good third party profibus scanners available).

Cheers Steve B.

Reply to
Stephen Bettesworth

Hi Pete,

Be very careful with 'large' networks when choosing a PROFIBUS master module. There is a big difference in theory and practice of PROFIBUS technology, caused mainly by the manufacturers of PROFIBUS DP masters.

The Mitsubishi PLC has a seperate PROFIBUS DP master module. That master module has a memory that it uses to make the image I/O data available to the PLC programmer. Many times that memory is limited to a few kilobytes (or even less) in which many times even all parameter and configuration setup data is also to be stored, leaving even less available for the really important stuff like input & output data. Depending on the number of slaves, the inputs, outputs, parameters, configuration and diagnostics that memory will fill up, an when it is full, and you still need to add more slaves, you have a problem. Then the only option is to add an additional master module and connect it to the same branch.

Please also do not confuse the number '32' as being a functional limit. This is a physical RS485 bus-load limit, easily bypassed by simply using readily available PROFIBUS RS485 repeaters.

Best regards,

Paulo Silva PROFIBUS Center Netherlands

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Paulo Silva


The number of slave stations is limited to 60 for Mitsubishi profibus DP masters. In mode E, the number of input and output bytes is flexible.

Hope this helps!

Regards, Sven

"Pete" skrev i meddelandet news:bsa9vr$62o$

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