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Modbus-S7 1200 and 11 Janitza powermeters -slaves no answers

Created by: luciferC4 at: 4/26/2010 4:03 PM (6 Replies)

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4/26/2010 4:03 PM	Rate (0)
luciferC4 Bronze Member Joined: 1/25/2006 Last visit: 2/9/2023 Posts: 257 Rating: (60)	Hi, I have been worked on one project for energy menagements. For aqusition of data for one power station I used S7 1200 cpu 1212C with comunicaton modul rs485, and 11 power meter as modbus slave devices. From all 11 devices I read and write the same parameters on the same locations. Concept of program is to use one FB block(I called modbus block)with 11 data instance blocks. In this FB block I used local variables and FB1081 for read and write data in slave, where two time slave is read and one time write data. In OB1 FB block( modbus block)called 11 times, in sequence. In OB100(startup) FB1080 block is called with parameters of comunication: no parity, 9600boudrate, 1000ms response time for slaves. Problem is that slaves no answers. None. In monitoring I can't see any mistake. Can someone advice me what to do. if it is necessary I can show program.
	Last edited by: luciferC4 at: 4/26/2010 9:26 PM Can someone tell me about termitation of bus, modbus? On side of CM1241 module. Is termination between 3 nad 8 pin or 6 and 8 pin? And which value of resistor should be use for termination? On side of the last janitza power meter termination is made with 120Ohm resistor.
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5/9/2010 9:40 AM	Rate (0)
luciferC4 Bronze Member Joined: 1/25/2006 Last visit: 2/9/2023 Posts: 257 Rating: (60)	Hi everyone, I solved my problem and I want to share with you. Maybe, someone will have the similar problem. The first, I tried to adjust parameters of modbus (baud rate, response time, parity, number of stop bits) but without success. The problem was in terminator (resistor). In manual I read that network must be terminated on both side. Of course, I listened that advice and I did termination with resistor 120Ohm. But as this post got name, none slave answered to master.In the end, in desperite attempt, I made program with one slave, in first, with resistor on slave side. But without success. Than, I decided to remove resistor on slave side and communication was established. After that I include all 11 slaves, of course, without resistor on slave side. Resistor on 9-pin conector on CM1241 exist!!! Why it was problem I don't know. Maybe, on short lenth of network it is not neccessary to put resistor for terminating of network. The second observation is in programming part this problem. My expiriance is that all modbus FB (FB1081)must be in one OB block or FC block. You shouldn't put in different blocks. Why? I haven't answer. And now, my network work well. Modbus parameters is: baudrate 38400, no parity, response time 800ms, one start bit, one stop bit. This is my experience. Maybe, if someone have similar problem, try to solve on this way. In the end, thanks all who read this post.

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6/17/2011 12:00 AM	Rate (0)
Saint Germain Experienced Member Joined: 8/18/2010 Last visit: 3/18/2023 Posts: 42 Rating: (9)	Hi LuciferC4, Thank you for sharing your findings! I have a program where I want to use Modbus to communicate to many slaves and I was wondering how you handled the sequencing. Since you can only have one Modbus message on at a time I assume you sequenced these triggers. Is it possible for you to share this portion of your program or show us how you handled this? Thank you in advance.

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11/11/2013 12:04 PM	Rate (0)
ionut.b Bronze Member Joined: 1/17/2007 Last visit: 10/6/2020 Posts: 289 Rating: (3)	Hello LuciferC4, I have a similar project with a number of 60 Janitza power meters (modbus rtu 485). I would like to read the data using S71200. Can you help me with an example of project programming ? S71200 can communicate on MODBUS RTU using CM1241 or CB1241. Any ideea what is the difference between the two options? Thank you.

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11/11/2013 1:41 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 12/6/2023 Posts: 1527 Rating: (528)	Termination is not always required, depending on cable run length and transmission speed. A good rule of thumb is "if the propagation delay of the data line is much less than one bit width, termination is not needed." It is explained very well in this white paper: http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/RS-422-and-RS-485-Applications-eBook/RS-422-RS-485-eBook-graphics-embedded.pdf. I can see why not fitting them when required would cause it to fail, however, I am a bit perplexed why fitting terminators when not required caused it to fail also.
	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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12/4/2015 3:04 PM	Rate (0)
Ziemens Silver Member Joined: 3/2/2007 Last visit: 11/8/2023 Posts: 895 Rating: (5)	is it the same issues with CB1241 ?
	... thank you ...
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3/30/2022 8:35 AM	Rate (0)
MW128 Regular Member Joined: 12/9/2020 Last visit: 12/5/2023 Posts: 17 Rating: (0)	Hi, LuciferC4 , What Address you entered in MB_Addr..? And what kind of data you were reading.
	Last edited by: MW128 at: 03/30/2022 08:36:44
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