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Modbus connectivity for CP441

Created by: Techy at: 3/8/2007 6:34 PM (10 Replies)

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3/8/2007 6:34 PM	Rate (0)
Techy Posts: 86 Rating: (2)	Hello, I am connecting PC to Siemens S7-400 PLC via CP441 & am facing problem while accessing the register memory area of Simantic. After double clicking the CP441 in the Hardware menu the Parameter screens pops up & I am stuck at the point of configuring the FC 03,06 Tab for Register read/ write access. It seems that the Simatic Memory area Words can't be directly read & has to be done through DB. How do I correlate the DB with the actual memory words in the Simatic memory area?? Do I need to create a DB in the S7? Any wise suggestions. Thanks, Techy
Techy Posts: 86 Rating: (2)
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3/8/2007 8:33 PM	Rate (0)
Tomcat Posts: 433 Rating: (48)	Hello Techy, do you use product "Open Modbus TCP" for the communication? If yes, then you need a DB. You have to create a DB and write your values in this DB. On Modbus-FB you have to give your DB-number and the adress. Tomcat
Tomcat Posts: 433 Rating: (48)
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3/8/2007 10:40 PM	Rate (1)
Late Platinum Expert Joined: 1/7/2006 Last visit: 11/17/2023 Posts: 2705 Rating: (318)	Hi, What do you have, a Modbus RTU Master or a Modbus RTU Slave? When it is a slave then for FC3 and FC6 you need a datablock that is mentioned in the slave parameters. There you have to give the first DB in the parameters. That DB then started on DBW0 for Register 1, DBW2 for Register 2 and so on. See also the handbook Loadable Driver CP 441-2 MODBUS Protocol RTU Format S7 is Slave You have also to program FB180 for the CP 441-2 to get the data transfert to the CPU and vice versa. See page 1-3 for the Holding Registers that are starting on 40001. Use of the Dongle Introduction In order to run the CP with loadable drivers, you require a dongle. When the dongle is plugged in, drivers can be loaded. Both interfaces of the CP 441-2 can operate with loadable drivers. How to Plug In the Dongle Before you can plug in the dongle, you must take the CP out of the rack. At the back of the CP, above the plugs for the backplane bus, there is a slot into which the dongle can be inserted. On page 3-3 you can see which dataformats can be used for which Modbus Function Code. On page 3-5 you will see the data for the holding registers. Access with Register-Orientated Function Codes Function Codes 03, 06, 16 The register-orientated function codes 03, 06, and 16 permit read and write access to the SIMATIC memory area data blocks. Calculation of the required data block number is carried out in two steps. 1) You can use the parameter assignment tool to specify a base DB number. This base DB is the first DB which can be accessed.2) The MODBUS Start_Register address (Register Number) transmitted in the message is interpreted as follows: (picture)
	I hope that the answer will help, if you have still questions, don't hesitate to ask me. All the best, Late Never say never, think in solutions, not in trouble......
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3/9/2007 8:28 AM	Rate (0)
Techy Posts: 86 Rating: (2)	Late Hi, What do you have, a Modbus RTU Master or a Modbus RTU Slave? Access with Register-Orientated Function Codes Function Codes 03, 06, 16 The register-orientated function codes 03, 06, and 16 permit read and write access to the SIMATIC memory area data blocks. Calculation of the required data block number is carried out in two steps. 1) You can use the parameter assignment tool to specify a base DB number. This base DB is the first DB which can be accessed.2) The MODBUS Start_Register address (Register Number) transmitted in the message is interpreted as follows: (picture) Late , thanks for the information.Yes it's a Modbus RTU Slave & I have configured the FB 180. Some more questions I am trying to read Simatic memory MW600 data. Do I need to build a DB1 –Assign MW600 data to DB1 Word 0; Or is the MW600 address passed directly to DB1 & no further programming from my side is reqd.. Is this correct? What is basic difference between a Holding register & Input register. The modbus address assigned for the registers is 40251 to 40260, but on FC03,06 tab there is no facility to enter the Modbus address, unlike the FC01 & FC02 which clearly specify the Range. How to correlate this to DB1. Could please elaborate on the Base DB number calculation u have mentioned above,with an example of say MW600 data; it will really help in clearing all doubts Thank you for the help. Regards, Techy
Techy Posts: 86 Rating: (2)
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3/11/2007 1:09 AM	Rate (1)
Late Platinum Expert Joined: 1/7/2006 Last visit: 11/17/2023 Posts: 2705 Rating: (318)	Techy I am trying to read Simatic memory MW600 data. Do I need to build a DB1 –Assign MW600 data to DB1 Word 0; Or is the MW600 address passed directly to DB1 & no further programming from my side is reqd.. Is this correct? What is basic difference between a Holding register & Input register. The modbus address assigned for the registers is 40251 to 40260, but on FC03,06 tab there is no facility to enter the Modbus address, unlike the FC01 & FC02 which clearly specify the Range. How to correlate this to DB1. Could please elaborate on the Base DB number calculation u have mentioned above,with an example of say MW600 data; it will really help in clearing all doubts Hi, On your questions I have the following answers: 1. A holding register is a register to place some value and a input register is a register where a input is coming true. 2. 40251 is coming into DB1.DBW((251 -1 )* 2), I have always to look is it coming directly in this address or one before or one behind . In a Datablock you might place 512 registers , then the next datablock is taken. 3. MW600 have to be placed into the datablock, so the instructions 'L MW600'and 'T DB1.DBW500' when you want to place it in register 251 (or one before/behind).
	Last edited by: Late at: 11.03.2007 01:10 some typeing errors corrected I hope that the answer will help, if you have still questions, don't hesitate to ask me. All the best, Late Never say never, think in solutions, not in trouble......
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3/15/2007 8:41 AM	Rate (0)
Techy Posts: 86 Rating: (2)	Late, One quick question- I have 200 registers (400) words. If my modbus start address is 40251 i.e. DB1.MW500 then how do I access data after 512 Words; Does it automatically move to next block or do I need to first create the DB2? Thanks, Techy
Techy Posts: 86 Rating: (2)	Last edited by: Techy at: 15.03.2007 10:32 Last edited by: Techy at: 15.03.2007 10:25
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3/15/2007 11:15 AM	Rate (0)
Techy Posts: 86 Rating: (2)	Late, How did you arrive at the calculation - *40251 is coming into DB1.DBW((251 -1 ) 2)? I was looking at the Modbus Slave loadable driver pt-pt PDF manual; on page 8-9 it shows following calculation forthe starting register 40251 (decimal) = 9D3B Hex. (D is the upper byte which is start register - Offset DB_No. which is = 9D or 157 dec. No to get the DB no. the equation shown is ( DB no. 1+ 157) = 158** is the resulting DB & the Data word DBW = (start register No. (the lower byte) = 3B or 59 dec * 2) = 118 so the first start word is DB158.DBW118 Can you tell me if my interpretation is correct / wrong? Thanks, Techy
Techy Posts: 86 Rating: (2)
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3/17/2007 1:08 AM	Rate (0)
Late Platinum Expert Joined: 1/7/2006 Last visit: 11/17/2023 Posts: 2705 Rating: (318)	Hi, I think your calculation is wrong. To test it, its not that difficult, ask for register 1 (40001) and check if it is getting from DW0 in DB1. When that is working you can ask for Register 512 and see if it is coming from word ..... If you ask for register 513 it is coming from DB2.DW0, that have worked fine for me. You have to make DB1 with a length of a minimum of 1024 and you have also to generate a DB2 of the correct length.
	I hope that the answer will help, if you have still questions, don't hesitate to ask me. All the best, Late Never say never, think in solutions, not in trouble......
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3/17/2007 1:12 AM	Rate (1)
Late Platinum Expert Joined: 1/7/2006 Last visit: 11/17/2023 Posts: 2705 Rating: (318)	Hi Techy, The 40000 is a number where the 4 is aspecially for registers, so don't calculate that to get the DB and DBW.
	I hope that the answer will help, if you have still questions, don't hesitate to ask me. All the best, Late Never say never, think in solutions, not in trouble......
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3/23/2007 11:49 AM	Rate (0)
Techy Posts: 86 Rating: (2)	Thanks Late, I was able to get it working Regards, Techy
Techy Posts: 86 Rating: (2)
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