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Modbus CP 441-2 (Master) <--> Wago 750-653 (Slave)

Created by: Morten Skelmose at: 4/16/2010 1:23 PM (5 Replies)

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4/16/2010 1:23 PM	Rate (0)
Morten Skelmose Posts: 4 Rating: (0)	Hi! I'm making a project right now, where i'm using a Siemens S7 400, (with Step 7 V5.4 and SP4). I have a 441-2AA04-0AE0 modbus module, with a master-dongle in it. The problem is, when i'm sending data to my WAGO-PLC, using RTU (both have the same interface parameters) then the data is "accepted" by the wago - i.e. i can change(write) coils and registers. But when the wago has to respond/echo backto the Siemens PLC, then the I/O module (750-653) has a 3 byte limitation - which means that the CP 441-2 module thinks that after the first 3 bytes have been recieved - the messages is "done" which means, that the Siemens PLC thinks that the echo is incorrect - and i get an error (code 2) An example: Slave: 1 FC : 6 Register adress: 6 Value: 255 CRC: xx xx Siemens Query 000000-Rx:01 06 00 06 00 FF 29 8B WAGO echo 000001-Rx:01 06 00 000002-Rx:06 00 FF 000003-Rx:29 8B i'm using the "Modbus Master" driver that came along with the dongle - and i've been using the sample-project as reference. I hope that there's someone who knows aboutthis problem? Thanks in advance
Morten Skelmose Posts: 4 Rating: (0)
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4/16/2010 3:47 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Hello Morten, how longis the delay between the third and fourth character of the respond? The standard character delay time of Modbus RTU is 3,5 times character transfer time, that means 3,5ms with 9600 Baud. I guess you have to increase the character delay time, as far as I know it is possible by a multiplication factor in the driver parameterization. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)
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4/16/2010 10:06 PM	Rate (0)
Morten Skelmose Posts: 4 Rating: (0)	Hi Kaulqappe! I'm not quite sure, since i haven't measured the time, butabout 100-200ms between eachtransmission. I've tried to increase the "Multiplier Character Delay Time" to its maximum (10), but didn't do any difference. Regards MortenSkelmose
Morten Skelmose Posts: 4 Rating: (0)
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4/17/2010 6:53 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Hello Morten, if the Wago device has gaps about 100ms between two characters within a telegram, it doesn't meet the Modbus specification. Probably you can ask Wago, if there is a new and faster driver for that slave. The last idea I have: reduce the baud rate.The transmission takes more time and the Wago device has got more time to execute the sending of the response. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)
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4/17/2010 8:55 PM	Rate (0)
Morten Skelmose Posts: 4 Rating: (0)	Hi Kualquappe, I will try that as soon as i can, probably monday, see if it helps. Thank you very much for your help. Regards Morten Skelmose
Morten Skelmose Posts: 4 Rating: (0)
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4/23/2010 3:38 PM	Rate (0)
Morten Skelmose Posts: 4 Rating: (0)	Well - now i found the answer - and it is kind of embarrissing, as i mentioned the time between the telegrams were about 100-200ms, and i measured it to excactly 200ms, and i couldn't find out, where in the program it said 200ms. Solution In CoDeSys V2,3you can assign the main program (ie. modbus)as a 'task' - and in here i can give the task a priority and a "scan interval" - and by default it had been set to 200ms.. so i just reduced it to 10ms, and now it works.. I hope that this can aid someone else in the same situation. Best regards Morten Skelmose
Morten Skelmose Posts: 4 Rating: (0)
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