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limitations of CP341 when used for Modbus RTU communication
Created by: Nigin at: 9/12/2012 11:03 AM (2 Replies)
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| 9/12/2012 2:41 PM | |
Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating:
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Dear Nigin, let me try to help you in the meantime. 1) The max. number of modbus slavesconneted toa CP341is254. Slave number 1-254, 255 is reservedfor broadcast requests from the master to all connected slaves. It is not only the number of slaves you need to considerwhen youthink about an acceptable performance. Important is also how many requests per slave are required to fetch/write all the data. So at the end you can say it's the number of requests in summary that counts. 2) I believe that there is a limitation but could not find any for that CPU at the moment. 3) Redundancy is not a problem but needs to be handled by your application. 4) Performance of the CPU is not a problem. The latest "flagship" is a beauty! Overall performance could be an issue.But when you consider point 1 youmake the best out of it. Other alternatives? Modbus TCP at the CPU withModbus TCP toserial converters. - Cheaper than CP341 solution -less space required compared to ET200M with CP's CP441-2 (pending on total number of interfaces required) - better performance through use of backplane bus instead of profibus to ET200M - less space, no ET200M rack required -slightly higher costscompared to CP341 When usingsingle CPU you would have more options but that'snotyour case, right?. Best Regards. |
Last edited by: TIBI68 at: 9/12/2012 2:45 PMGive us your feedback and show us your respect! |
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| 9/12/2012 2:53 PM | |
Joined: 9/27/2006 Last visit: 3/18/2025 Posts: 12344 Rating:
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Hello Nigin; Let's see if we can provide some answers to your questions.
The main manual for the CP 341 as Modbus master is SIMATIC S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is master This quote is taken from the presentation of the protocol, in Chapter 2: With this driver, it is possible to use the RS422/485 (X27) interface submodule in both 2-wire operation and 4-wire operation.In 2-wire operation it is possible to connect up to 32 slaves to one master in half-duplex operation, thus creating a multipoint connection, a network. What properties, advantages and special features does the Modbus protocol offer?
To use the CP 341 with a S7-400h system, you will need to insert it as part of a redundant distributed Profibus station (ET200M with dual IM 153-2 and active bus modules): How do you download a loadable driver such as "Modbus Master (RTU)" into a CP341 that is connected to an S7-400 H system by means of an ET 200M? You must then calculate the number of CP 341 used in each ET 200M station based on the fact that a Probibus slave (the ET 200M is a Profibus slave) is limited to 244 bytes Out/244 bytes In.
As part of the redundant ET 200M you would have redundancy at the network level: ifthe primaryS7-400H CPU goes down then the standby CPU will continue reading data from the Profibus networks and the redundant ET 200M stations. However Modbus as a protocol does not support redundancy natively: only one Modbus master can be active at a time on a Modbus network. We have used relays controlled by the PLC outputs,to switch between 2 Modbus masters when the PLC is aware of the loss of one master, but this is a stop-gap solution at best, and only works at the very lowest Modbus speeds (9600 Baud) to allow the relays' switchover time.
Well, Modbus is quite robust, which is why it is still used and appreciated in the automation worlddespite its low speed and cycle time. If the data collection time is not critical for your application (monitoring of process data only), then it should work, if you have enough programming time and ressources. If the data is required at a higher rate (for alarming or control purposes), Modbus TCP solutions come to mind, or maybe even Profibius-DP, through gateways of interface cards available for your slaves. Hope this helps, Daniel Chartier |
Last edited by: dchartier at: 9/12/2012 2:57 PM |
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