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Modbus Slave (RTU) understanding

Created by: wachman at: 1/11/2008 12:34 PM (5 Replies)

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1/11/2008 12:34 PM	Rate (0)
wachman Advanced Member Joined: 8/29/2007 Last visit: 6/9/2022 Posts: 28 Rating: (1)	Hello, I had to add to a PLC program a MODBUS communication, My PLC will be Modbus Slave (RTU), I've got modbus address from my communication partner, I have also a tag list, where every tag has a modbus address, What is this address ?? How to understand this ?? eg. temp_tag1 - modbus address: 1 temp_tag2 - modbus address: 2 And as I understand (if I'm wrong please correct me): if my PLC is a slave then only things I must to do is copy FB7, FB8 and FB80 to my project, call cyclically FB80 and correct configure modbus-addresses conversion to enable read my DB's -> Modbus Master will request some data and my Modbus Slave will answer

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1/11/2008 1:36 PM	Rate (0)
PeteB Posts: 16 Rating: (2)	As well as calling the relevant FB80 you need to setup the Modbus Slave protocol on the CP341 in the Step7 Hardware Configuration editor. For each Modbius Function Code (FC) there is a tab to allow you to map the Siemens memory to a Modbus Address e.g. The tab "FC 01,05,15" allows you to define which Modbus Address's map into Memory bits (M), Outputs (Q), Timers (T) and Counters (C). The tab "FC 03, 06, 16" allows you to define which Modbus Address's map into the DB's. There aretabs for comms in the opposite direction as well. The manual that is installed on your harddisk with the Modsbu Slave driver software covers it quite well.
PeteB Posts: 16 Rating: (2)
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1/11/2008 3:45 PM	Rate (0)
wachman Advanced Member Joined: 8/29/2007 Last visit: 6/9/2022 Posts: 28 Rating: (1)	Has anyone know some good program (freeware) to simulate MODBUS Master on PC ??

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1/11/2008 3:49 PM	Rate (0)
PeteB Posts: 16 Rating: (2)	The kepware OPC Server covers a vast array of protocols. Very useful for testing comms links. http://www.kepware.com/
PeteB Posts: 16 Rating: (2)
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1/12/2008 3:46 PM	Rate (0)
Late Platinum Expert Joined: 1/7/2006 Last visit: 12/23/2024 Posts: 2705 Rating: (320)	Hi Pete, 795$ is not freeware I think.... Hi Wachman, I have found a freeware Modbus RTU Serial / Modbus RTU over Ethernet \ Modbus TCP over Ethernet Simulator for test purposes tool. QuickMod Modbus scanning tool I didn't test it because I don't have a Modbus slave here, I have placed the desription of the program in the attachement, you may download it from the link above, it is20Mb. Attachment QuickMod Modbus scanning tool.pdf (662 Downloads)
	Last edited by: Late at: 12.01.2008 16:01 I installed the option runtime only, it started well after sometime installing, I choose the option "QuickMod - Modbus scanner" 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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1/14/2008 10:33 AM	Rate (0)
wachman Advanced Member Joined: 8/29/2007 Last visit: 6/9/2022 Posts: 28 Rating: (1)	@Late, thanks, QuickMod Modbus scanning tool works very well, but.... I send a request: Tx (09:25:33.269): \x05\x03\x00\x00\x00\x04E\x8D (address 5, function 3, DB1, DBW0), base DB number = 1, but I get replay: Rx (09:25:33.298): \x05\x83\x04\x012 (address 5, function should be 3 (not 83), 04 - I think this is byte count, 012 - I think this is CRC, but where is DATA?? ) what may be wrong?? ------------------------------------------------- OK, FB80 returns an Error_Nr = 92, what means: "Error when executing a RECEIVE/FETCH call with FB7 (RCV_RK).", Remedy: "Analyze FB7-STATUS", but I can't get FB7 status because FB7 is protected..... ------------------------------------------------ OK, finally it "works" (in yellow area shows current values), but when i press monitor button and analyze frames it looks strange and still is the same error code in call FB80, I hope it will work in real device.
	Last edited by: wachman at: 14.01.2008 11:58 Last edited by: wachman at: 14.01.2008 10:40
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