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CP341 programming FB8

Created by: Lego_PLC_Wesley at: 2/8/2009 5:10 PM (3 Replies)

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2/8/2009 5:10 PM	Rate (0)
Lego_PLC_Wesley Posts: 3 Rating: (0)	I'm also configuring a C341 via RS485. I have some questions in using the function codes. Wondering if anyone can help in answering them. I'm trying to communicate to a SLAVE device. 1. Where does the Modbus function code come in on FB8? There's a DB42 containing the function code in the sample program. In the sampe program, the only reference to it is in FB8's DB_NO := 42. 2. What/Wheredoes LADDR (Logical Address) refer to? 3. What does DB_No and DBB_No mean and refer to? Thanks in advance......
Lego_PLC_Wesley Posts: 3 Rating: (0)	Last edited by: CS Moderator at: 2/8/2009 10:37 PM new subject after splitting
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2/9/2009 5:52 AM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/11/2023 Posts: 12193 Rating: (2638)	Lego_PLC_Wesley I'm also configuring a C341 via RS485. I have some questions in using the function codes. Wondering if anyone can help in answering them. I'm trying to communicate to a SLAVE device. 1. Where does the Modbus function code come in on FB8? There's a DB42 containing the function code in the sample program. In the sampe program, the only reference to it is in FB8's DB_NO := 42. 2. What/Wheredoes LADDR (Logical Address) refer to? 3. What does DB_No and DBB_No mean and refer to? Thanks in advance...... Hello Wesley; Here are some quickanswers to your questions. More detailed information can be found in the manual "Loadable Drivers... Modbus Protocol, RTU Format, S7 is master"; it can be found in the link below. 1. The function code is loaded in the instance datablock for the FB8 ("FB P_SND_RK"), in the example programin DB50; look at chapter 5, Function Codes to see in which DB register it needs to be located. 2. LADDR is used to tell the CPU which I/O address it can use to connect to the CP on the rack. You can use the first address given to you by the HWConfig editor when you setup the CP 341. 3. DB_No (number of the DataBlock) and DBB_No (number of the DataBlock Byte), along with the LEN(gth) parameter are used to build a pointer to the data you neen to send. In which DB is it located, at what byte inside this DB does it start, how much data (the number of bytes) you want to send? Modbus Master manual for the CP341: http://support.automation.siemens.com/WW/view/en/1220184 Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 2/9/2009 5:53 AM
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2/9/2009 6:15 AM	Rate (0)
Lego_PLC_Wesley Posts: 3 Rating: (0)	Thank you, dchartier ...... One question. Looking inside the sample prgoram, DB50 is the instance DB for the FB. Moreover, DB42 contains the following data declaration: Slave Address (Byte); and Fuction Code (Byte); On the STL sample program, DB42 is used at DB_NO: Is this where the Source DB is placed? Wesley
Lego_PLC_Wesley Posts: 3 Rating: (0)
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2/9/2009 6:27 AM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/11/2023 Posts: 12193 Rating: (2638)	Hello Wesley; In the case of the example program, DB42 is where the source data to be sent is located. Or, as described in the manual: FC 21 Execute Send JobsFC calling SFB P_SND_RK (FB8, DB50) DB 42 SOURCE_DB P_SND_RK Source DB with send data So when FC21 calls FB8, this one executes the SEND function, and sends the dataand the Function Code defined in DB42, starting at DBB 10 (so the start address for theSEND data is DB42.DBB10), with a data length defined in DB40.DBW10. Sounds complicated at first, but work at it until you understand the logic of the different command areas for the control of the Modbus master interface. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 2/9/2009 6:30 AM Last edited by: dchartier at: 2/9/2009 6:28 AM
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