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CPU 313C + CP 341 (modbus slave)

Created by: Ton Verra at: 9/15/2010 12:00 PM (6 Replies)

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9/15/2010 12:00 PM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 4/18/2025 Posts: 728 Rating: (43)	Hello, A couple of short questions, I hope . I got the hardware as stated above. Already used it in an other project, but only with sending data. Where do I state how many bytes I send to the master? And from where (what DB)? Where do I state how many bytes I receive from the master? And to where (what DB)? How many bytes can I send and receive? As always, thanks for all the help here, you guys (and gals, I think) rock! Ton

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9/15/2010 12:35 PM	Rate (1)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 4/18/2025 Posts: 8956 Rating: (1194)	Dear Ton Verra, Here is a link to manual: SIMATIC S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is slave The masterrequests the data and sends data it wants. There must be some agreement with the Master about the Data area's you want to use. Hope this will help. Best regards, Wizard

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This contribution was helpful to 1 thankful Users Ton Verra

9/15/2010 1:00 PM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 4/18/2025 Posts: 728 Rating: (43)	Hi Wizard, Thank you very much! But how much data can I send and receive? And how do I state how much I am sending? Ton

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9/15/2010 1:32 PM	Rate (1)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 4/18/2025 Posts: 8956 Rating: (1194)	Dear Ton Verra, When you have a look in manual, see chapter 5.6.2 for more details about the different function codes. The master request the data, by sending the information he wants. The only thing you should do is, move the desired data to the "communication area" Par example some DB's Chapter 9, will also give more detailed information about the Function Codes Best regards, Wizard

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This contribution was helpful to 1 thankful Users Ton Verra

9/15/2010 1:58 PM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 4/15/2025 Posts: 1560 Rating: (540)	Ton Verra But how much data can I send and receive? This depends on the datatype you are using:- For holding registers (address 40001+ using function codes 3, 6 & 16) and input registers (address 30001+ using function code 4) the master can send (write) or demand (read) 127 registers. Registers are 16-bit (2 bytes). For coils (address 00001+ using function codes 1, 5 & 15) and discrete inputs (address 10001+ using function code 2) the master can send (write) or demand (read) 2040 boolean bits. Boolean bits are 1-bit (you get 8 to each byte). Ton Verra And how do I state how much I am sending? As a slave device you do not specify how much you are sending. The master device does that. The slave is totally passive and will do absolutely nothing unless it receives a requeat from the master. This request contains the ID of the slave that the message is for, the function code (which defines what action it wants the slave to perform), the start address of the data it wants to read/write, and the ammount of data it wants to read/write.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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This contribution was helpful to 1 thankful Users Ton Verra

9/15/2010 2:51 PM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 4/18/2025 Posts: 728 Rating: (43)	Thanks both.. I will have to try.. Last time I got it to work, without really understanding it. I don't like that. We will see, I'll let you know if it works!

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This contribution was helpful to 1 thankful Users Wizard

9/15/2010 3:10 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 4/15/2025 Posts: 1560 Rating: (540)	Well it doesn't have to be that way. You can ask questions here as to how modbus works and you will get some answers. I am an expert in all things modbus. Ask away. I would sooner you know how things work rather than just getting lucky.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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This contribution was helpful to 2 thankful Users Wizard Ton Verra

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