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problems with Modbus RTU: the slave responds with code 03 ( Illegal data value)

Created by: XmEsNiX at: 12/2/2009 5:13 PM (3 Replies)

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12/2/2009 5:13 PM	Rate (0)
XmEsNiX Posts: 4 Rating: (0)	I have problems with Modbus RTU on S7-1200. I am using PLC as a modbus slave and PC as Modbus master. All functions (01,02,04,05,06,15,16) are working fine. The only problem is function 03 ( Read holding register). Everytime I try this function the slave responds with code 03 ( Illegal data value). Status output from MB_SLAVE block is 8383. The program is written in the same way as a example found somewehere on this forum. Does anybody have any idea what I'm doing wrong? Thank you for replies.
XmEsNiX Posts: 4 Rating: (0)	Last edited by: XmEsNiX at: 12/3/2009 3:21 PM Last edited by: O_Moderator at: 12/3/2009 10:14 AM new subject after splitting from /tf/WW/en/Posts/36132#top
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12/3/2009 4:29 PM	Rate (0)
XmEsNiX Posts: 4 Rating: (0)	I did some further testing and I found out that I get Ilegal data value error if I am running PID in one of the Cycle interrupt OB's. This also happends if the MB_Slave and PID_Compact block are called from the same OB. If I remove PID_Compact block call from OB, Modbus communication works without problems. No idea though how to solve this.
XmEsNiX Posts: 4 Rating: (0)
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12/3/2009 5:41 PM	Rate (0)
J_Bell Posts: 5822 Rating: (411)	Hello, well... I am not the MODBUS specialist...but what you say sounds rather like overwritten data areas...maybe of the local-data stack.Have you ever tried to put either of the two functions to an FB or FC called in this OB.... to get another local-data area... maybe this will fix it already. But it is only a guess. Did you contact Customer Support about it already? Best regards J_Bell
J_Bell Posts: 5822 Rating: (411)
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12/4/2009 8:31 AM	Rate (0)
XmEsNiX Posts: 4 Rating: (0)	Calling Mobus function from FC solved the problem. Thank you for your help.
XmEsNiX Posts: 4 Rating: (0)
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This contribution was helpful to 2 thankful Users J_Bell how come

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