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Could I just use the FC03,06,16 tab, and send my bits as an word?

Created by: Ton Verra at: 9/21/2010 11:10 AM (4 Replies)

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9/21/2010 11:10 AM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 3/4/2025 Posts: 728 Rating: (43)	Well, here I am again... I got a list from my costumer. I have to fill in function code, register adress + +. The other side of the modbus line is also Siemens, but I think its Desigo, and he knows nothing about S7 300... And I'm stuck, I really don't see any logic in this. See attachment. I filled a datablock with the data I want to send, the other one is ready for receiving. Why do I need to set memory bits, outputs, timers and counters? Could I just use the FC03,06,16 tab, and send my bits as an word? Or doesnt that make sense? Could someone please help me with this? I feel kinda stupid here... Ton Splitted from CPU 313C + CP 341 (modbus slave). Attachment modbus, slave.pdf (179 Downloads)
	Last edited by: O_Moderator at: 9/21/2010 2:16 PM new subject after splitting
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9/21/2010 2:27 PM	Rate (1)
Kaulquappe Posts: 3093 Rating: (324)	Hello Ton, in your data block you mixed up register oriented and bit oriented values. That is not really "Modbus like". Modbus communication refers to 4 different memory areas / types: coils, inputs (both bit oriented) and holding register and input register (both word oriented). The function codes describe which memeory area is accessed. In your case it depends whether the master wants to read/write single bits or always the whole area (or amultiple of16 bit). If the master accesses always a multiple of 16 bit you can put them in a register oriented memory area. Then I'd recommend to set "base DB number" to 24. Then the master can read from register address 0 and has to write from register address 512. Please note: a master writes and reads (active) and a slave receives and answers (passive). Regards, Kaulquappe Hello Ton, the moderator splits the thread during my answer. So it sounds a little bit akward.
Kaulquappe Posts: 3093 Rating: (324)	Last edited by: Kaulquappe at: 9/21/2010 2:32 PM
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This contribution was helpful to 1 thankful Users Ton Verra

9/21/2010 9:16 PM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 3/4/2025 Posts: 728 Rating: (43)	Thank you very much! Would it better to sort the bits and words? Or is it better to transfer the bits to a memorybit area? And sorry to ask, but where did the 512 come from?

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9/22/2010 11:35 AM	Rate (1)
Kaulquappe Posts: 3093 Rating: (324)	Hello Ton, I personally don't like DBs which start with BOOL, continue with INT and return to BOOL - especially in the context with Modbus communication. The bit values which are read by the master can be located in a DB besides INT values. When the master prefers function code 16 to write values, you can put the bit values in a DB too. But you have to take into account, that always 1 word (16 bit) will be written. So you should group bits to a wordwhich are written at the same moment. You can alsohave unused bits in such a word. When the master uses function code 15 to write bits, I recommend to map them tothe flags of S7. Function code 15 supports to write single bits. The Modbus slave driver / function block FB80 start with Modbus register number 0 when taking about parameterization. Youhave the values the master should read in DB24. Therefore you should set base DB to 24, and the master starts reading at Modbus register address 0. In the S7 512 registers are mapped to one DB. As the values the master will write are in DB25 it corresponds to Modbus register address 512. (If the values to write were in DB 26, the master must use register address 1024 - just as an example) Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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This contribution was helpful to 1 thankful Users Ton Verra

9/22/2010 4:42 PM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 3/4/2025 Posts: 728 Rating: (43)	I'm starting to get the picture now... Early start tho.. Why can't they make modbus a bit easier? hehe... It annoys me that I can't safe the configuration tho...

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