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CM1241 as modbus master

Created by: Darius at: 8/4/2009 2:11 PM (6 Replies)

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8/4/2009 2:11 PM	Rate (0)
Darius Platinum Member Joined: 9/28/2005 Last visit: 12/10/2024 Posts: 2957 Rating: (243)	Hello, It is said in the catalog that this modules uses point-to-point communication. So if I have CM1241 with RS485 interface set as modbus master then how many slaves can I connect ? Is it only 1 ?
	Best regards, Darius
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8/4/2009 5:59 PM	Rate (0)
Denilson Pegaia Platinum Member Joined: 9/23/2005 Last visit: 1/17/2025 Posts: 4394 Rating: (1463)	Hi, this is not a question of hw limitation. It is just a question of a user program and the performance of this user program.
	Denilson Pegaia Siemens Industry Sector Technical Support
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8/5/2009 2:44 PM	Rate (0)
bakerjw Posts: 126 Rating: (18)	There is an electrical limitation to how many devices that can be connected to the RS485 port before the signals degrade. I'd have to look to see how many devices that it is.To go beyond that number would require the use of a repeater. Other than that the Modbus master on the S7-1200 can access Modbus slaves from 1 to 247 without any issues. During testing we were able to access 247 slaves on one network with a modified Slave program..
bakerjw Posts: 126 Rating: (18)
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8/5/2009 6:21 PM	Rate (0)
Denilson Pegaia Platinum Member Joined: 9/23/2005 Last visit: 1/17/2025 Posts: 4394 Rating: (1463)	Hi, you are right: the RS485 has a limitation of 32 participants per segment. However repeaters could be use to extend this.
	Denilson Pegaia Siemens Industry Sector Technical Support
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8/5/2009 7:56 PM	Rate (0)
Darius Platinum Member Joined: 9/28/2005 Last visit: 12/10/2024 Posts: 2957 Rating: (243)	Pegaia Hi, this is not a question of hw limitation. It is just a question of a user program and the performance of this user program. If you look at the info of CM1241 you will see that it has implemented modbus protocol which runs on PtP link. So what does it means ? If I understand correctly it has integrated modbus protocol in hardware and no library is needed. How to understand "runs on PtP link" ? Maybe its integrated modbus protocol only support PtP connection only with one device ?
	Best regards, Darius
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8/6/2009 3:01 PM	Rate (0)
bakerjw Posts: 126 Rating: (18)	The wording may be confusing. The Modbus protocol is not built into the hardware. To use Modbus RTU on a PtP link, you do need to use the Modbus library. The library consists of 3 different components. MB_COMM_LOAD, MB_MASTER, and MB_SLAVE. MB_COMM_LOAD is executed a single time to configure a single serial port that will be used for either the MB_Master and MB_Slave components of the library.
bakerjw Posts: 126 Rating: (18)
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This contribution was helpful to 2 thankful Users Darius Wat_HTC

6/3/2014 11:47 PM	Rate (0)
CS Moderator Moderator Joined: 9/1/2005 Last visit: 1/17/2025 Posts: 4114 Rating: (195)	follow up question published by AutoGen_283349is split to a seperate thread wiht the subject CM1241 as modbus master - What licences are required Best regards CS Moderator
	My Forum is the place to personalize your profile, track your topics, and maintain content that interests you. Best regards CS Moderator
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