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cp341-modbus master

Created by: Solution Riders at: 8/26/2010 4:22 PM (8 Replies)

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8/26/2010 4:22 PM	Rate (0)
Solution Riders Posts: 112 Rating: (0)	Dear experts, I have workiing with CP341 MODBUS MASTER and s7-300 plc. Communication established between CP341 and UPS.But i getting following error reading in destination data block DB43. B#16#FF. I dont know whereexactly problem found?... Imattached here ups address map and modbus errorscreen shots for ur reference.. Kindly give us solution.. Thanks in advance... Attachment modbus error.zip (215 Downloads)
Solution Riders Posts: 112 Rating: (0)
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8/26/2010 8:31 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/31/2025 Posts: 12479 Rating: (2722)	Hello Solution Riders; It would seem that the slave does not recognize your Modbus query, and is sending back a garbled message. How exactly are you requesting data say, for address 30004 in the slave (the number of lines)? Try reading only one register for starters, it is then easier to debug the slave's reply. Hope this helps, Daniel Chartier

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8/27/2010 6:37 AM	Rate (0)
Solution Riders Posts: 112 Rating: (0)	Dear dchartier, Thanks for ur reply.That modbus address 30004 is decimal value.I convertdecimal 30004 to hexa decimal7534.so i given 7534 is starting address of the modbus.any other option available for giving starting address?..... And i written query like that... L B#16#2 T "SOURCE_DB".slave_address L B#16#4 T "SOURCE_DB".function_code L W#16#7534 T DB42.DBW 2 L W#16#A T DB42.DBW 4 L W#16#6 //Length SEND-Data T DB40.DBW 10 //SEND LEN CALL "Execute Send Jobs" //FC 21 CALL "Execute Receive Jobs" // FC 23 Kindly give us solution. Thanks in advance...
Solution Riders Posts: 112 Rating: (0)
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8/27/2010 10:32 AM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 11/20/2025 Posts: 1561 Rating: (540)	I am sure that the starting address is an offset from the base address of the database not the actual address. You have done what a lot of people have done with modbus in the past and confused offset and address. An offset of 0 gets you the first address of the database relating to the chosen function code. So offset 0 will get you:- 1) Coil address 00001 (for function codes 01,05 & 15) 2) Discrete Input address 10001 (for function codes 02) 3) Input Register address 30001 (for function codes 04) 4) Holding register address 40001 (for function codes 03,06 & 16) So in your case the value 16#7534 (30004) should actually be 30004 - 30001 (base address for Input Registers) = 3.
	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 dchartier

8/27/2010 4:41 PM	Rate (0)
Solution Riders Posts: 112 Rating: (0)	Dear smiffy, Thanks for reply... Im new one for this concept.I dint understand wat above u mentioned.Dont mistaken me. Can u tell me which value can i enter in starting adress instead of b#16#7534. Imusing function code 4. I want to read input register.. Kindly give us solution.Imin bottle neck.. Thanks in advance....
Solution Riders Posts: 112 Rating: (0)
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8/27/2010 5:17 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/31/2025 Posts: 12479 Rating: (2722)	Hello Solution Riders; You wish to read data in Modbus address 30004. By requesting Modbus Function 4, you specify that the addresses you want to reach register addresses starting at 30001, start of the ModbusInput Registers range(Function 4 = Read Input Registers). So the Modbus master knows your base register address. You then need to specify the offset address from 30001. As pointed out by Smiffy, try w#16#3... itshould take you to 30004. Hope this helps, Daniel Chartier

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This contribution was helpful to 1 thankful Users smiffy

8/28/2010 4:22 PM	Rate (0)
Solution Riders Posts: 112 Rating: (0)	Dear smiffy and Dchatrier, As per ur suggestion i did logic.Now modbus working well.. Thank u so much..Keep it up....
Solution Riders Posts: 112 Rating: (0)
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8/28/2010 5:18 PM	Rate (0)
Solution Riders Posts: 112 Rating: (0)	I have one problem...I want read data from genset.. genset Vendor was gave modbus address details.In that details Modbus word address starting from decimal 258, 260,264,180..like that In this casewatkind of conversion required?..Which value i have to enter in starting addressof modbus logic.Here i with attached my Genset Modbus details L W#16#.......? //Starting address of genset T DB42.DBW4 Kindly give us solution.. Thanks in advance.... Attachment ModBus_communication_generator.zip (226 Downloads)
Solution Riders Posts: 112 Rating: (0)
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8/29/2010 2:12 AM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/31/2025 Posts: 12479 Rating: (2722)	Hello Solution Riders; When you request data from a slave with Modbus Function 4, you specify the start register address in the slave and the number of registers to be read. Then in your Recieve DB you get as a reply the data of the first register, then the data of the second... This is where you find your data. Say you want to read registers 256, 258 and 260 in your slave. Put startregister as number 256, and ask for 5 registers. Your Recieve DB will contain 5 datas, the first(DBW0) for slave regiister 256, the third (DBW4)forslave register258 and the fifth (DBW8)will contain data for slave register260. The data in DBW2 (257) and DBW6 (259)is irrelevant toyou in this mapping; but it is easier to request a simple range of data from the slave than to do a number of requests for specific slave registers.. Once you have transfered this data to a user memory space, you can look at the specified bits as you wish. Note that you will have to do a second request for the higher data registers (above offset 4xxx) as you cannot request thousands of data registers at a time. The limit the CP 341 will handle is 127 registers per request, as you can see in the Modbus drivermanual, Modbus Function Codes (Ch.5). Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 8/29/2010 2:17 AM
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