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Error in Write single Coil for Modbus RTU CP341 in S7-400 system

Created by: Gianni_BT at: 9/11/2014 11:35 AM (10 Replies)

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9/11/2014 11:35 AM	Rate (0)
Gianni_BT Posts: 6 Rating: (0)	We have 2 CP341 loaded with Modbus RTU slave driver in 2 different IM153-2 Racks connected to a S7-400 redundant system based on CPU414-5H PN/DP CPUs Firmware Version 6.0. The loadable driver is Modbus Slave RTU V.3.1 SP9. All Modbus messages work correctly except the "write single coil" (Modbus code 05): the Modbus return message has a code HEX 85(exception return message) and an error code 4 (unrecoverable error in slave while performing the action). The same problem happens on 2 old systems, with same CP341 and CPU S7-400 CPU412 H and apperars using 3 different Modbus Master systems. Notice that the Read single coil (Modbus code 1) on the same merkers work correctly and operation on registers (Modbus codes 3/6/16) work perfectly. The addressed coils are within the Limits configured in the protocol configuration Tabs (both FC01,05,15 Tab and Limits Tab). Is there any other parameter or settings that can prevent the CP341 to write in the PLC CPU?
Gianni_BT Posts: 6 Rating: (0)
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9/11/2014 11:41 AM	Rate (0)
Saravana kumar Gold Member Joined: 2/6/2009 Last visit: 1/7/2025 Posts: 939 Rating: (118)	Hi, Which is your slave divice? Is it possible to upload the manual of it?
	_______________________________________________________________ Best Regards T.Saravana Kumar "All for the Best"
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9/11/2014 12:39 PM	Rate (0)
Gianni_BT Posts: 6 Rating: (0)	Hi, the slave(s) are the CP341 that are equipped with Modbus RTU Slave driver. The test was done with 3 Modbus Master programs: ModScan32, Honeywell DCS another italian supervisory system driver: in all cases the result is the same.
Gianni_BT Posts: 6 Rating: (0)
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9/11/2014 3:05 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	Have you checked that the DB you are trying to write to is not protected or unlinked? Just a thought.
	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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9/11/2014 4:42 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Gianni, which data type in the S7 do you address - flags, outputs, timer or counter? Timer and counter can't be written with FC5. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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9/11/2014 9:28 PM	Rate (0)
Gianni_BT Posts: 6 Rating: (0)	We are triing to write merkers area( M3000.0 to M3001.7 ): these merkers are not used anywhere else in the program and can be written using a VAT. The modbus protocol can read these coils with Read Coil message ( Modbus code 1) , but cannot write them( Modbus code 5) : any attempt to do it has the result of the Exception error described at the beginning of the discussion.
Gianni_BT Posts: 6 Rating: (0)
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9/12/2014 8:58 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Gianni, then there must be an parameterization error in the LIMITS tab. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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9/12/2014 2:15 PM	Rate (0)
Gianni_BT Posts: 6 Rating: (0)	In the "Limits" tab of the Modbus slave protocol the limits are set in this mode for Memory bits: MIN 1000.0 ; MAX 3999.7. So, the area we are triing to write is within the limits. Probably if limits are not correct, also the read function has problems(I am not sure about this). Another strange thing is that using FB80 V. 1.4 the answer to the request to write a single coil is an exception message as described at the beginning of this discussion, while using the FB80 in version 1.6 ( the last available in the Modbus package V. 3.1 SP 9), there is no answer at all to a single coil write message. All the other Modbus function code , including single coil read,work properly both with V. 1.4 and V. 1.6
Gianni_BT Posts: 6 Rating: (0)
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9/12/2014 2:31 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Gianni, the LIMITS tab is only relevant with writing requests of the master. Reading is possible for all addresses defined in the FC1/5/15 tab. As you found differences in reaction of the CP I recommend to start a service request. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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9/15/2014 7:51 AM	Rate (0)
Gianni_BT Posts: 6 Rating: (0)	The service request was issued through the Italian support call Center 6 days ago, but, at the moment, no reply from that side. Thanks anyway for your support Regards
Gianni_BT Posts: 6 Rating: (0)
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