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| 1/4/2017 4:22 AM | |
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Posts: 8 Rating:
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In my case ,I connected the a1,b1 to +/- of a modbus device. both are using baud rate 9600 ,8N1. When I am polling to get the data , CM1241 only Rx Led is flashing,and I checked the mb_slave db. It's no error status, and only receive bad crc continuously. Do I miss something about hard ware configuration or communication setting? |
| 1/4/2017 4:41 AM | |
Joined: 2/27/2014 Last visit: 2/18/2025 Posts: 689 Rating:
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Hello, Sorry but half information can get only half solution. Which PLC version? Which software version? Is it MB Master or MB slave? Who is partner? Have you checked partner is ok or not? You can find so many modbus rtu sample application on Siemens support or forum. Write small logic to get status information. Move the MB Master/Slave status in other memory location while error bit is high. Post the outcome of status code here ,so expert can suggest you necessary changes. |
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With Best Regards, |
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| 1/4/2017 4:56 AM | |
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Posts: 8 Rating:
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SW,TIA V13 SP1 |
| 1/4/2017 10:26 AM | |
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Joined: 2/27/2014 Last visit: 2/18/2025 Posts: 689 Rating:
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Hello, Now information looks good. Let see Scenario, S71200 acting as Modbus Slave. use Modscan software for testing.( Modbus Master) As i suggested, Write small logic , while getting error bit of Modbus Slave block move the status in some other memory location. Then let us know the status code. see the attached picture. AttachmentS71200.zip (355 Downloads) |
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With Best Regards, |
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| 1/13/2017 10:54 AM | |
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Posts: 8 Rating:
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I tried the modbus_slave and this remain in status 8280. |
| 1/13/2017 12:57 PM | |
Joined: 4/28/2015 Last visit: 2/24/2025 Posts: 10832 Rating:
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Hello, very sry, now totally lost idea who is communicated with who. Can you pls again clearly give info about Master device and Slave device? Communication parameters we know, 9600/8/N/1. If Master is PC application, which type? Is used some adapter/converter? |
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With best regards... |
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| 1/13/2017 1:46 PM | |
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Posts: 3093 Rating:
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When you receive bad CRC constantly, there is a general error in parameterization or wiring. Did you use the same parameterization of the converter and the same wiring when the S7 acts as master (which worked obviously)? If yes, the parameterization at S7 side is wrong when using the slave program. Regards, |
| 1/14/2017 8:26 PM | |
Joined: 3/28/2010 Last visit: 3/13/2025 Posts: 1123 Rating:
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Since you already have the Modpoll 3rd party PC master, you might consider reading some non-zero data from the slave device so that you can observe whatever feedback (blinking LEDs) the RS-485 port has for the slave device and gain the confidence that the slave does indeed talk Modbus. There are generally fewer things to set up or to go wrong with the generic masters, so it's easier to get the comm up and running. Doing so can also tell you whether the addressing Modpoll uses is offset by one, where address 0032 = (4)0033 Look for a non-zero value because lots of slave devices zero out unused register so you might get a zero reply even though the data is from an address offset by one, and the real data is not zero. If a Receive LED blinks, then the A/B wires are probably not crossed, but a common issue with RS-485 is that the A/B lines are labeled the opposite by different vendors. Swapping the A/B lines on one end fixes it. Backwards installation will not damage the line drivers, but communications will never work. |
| 1/16/2017 3:53 AM | |
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Posts: 8 Rating:
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After swapping A/B lines , the miracle occur!!!! I can use modbus poll to get the data. |
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| 1/16/2017 2:26 PM | |
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Posts: 3093 Rating:
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It is always + to + and - to - , irrespective of the name of the signal. |
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