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Modbus RTU set up correctly

Created by: universalanalyzers at: 12/2/2014 12:16 AM (8 Replies)

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12/2/2014 12:16 AM	Rate (0)
universalanalyzers Posts: 7 Rating: (0)	Hello Everyone, I was hoping someone could provide me some guidance. I have attached a picture for how I have set up my system for modbus rtu. I cannot seem to draw the data out of the system. I believe it is an issue with how my DB is setup. I am trying to use the same DB for serial as I am currently using for TCIP. Which is probably a mistake. My modbus over TCIP works like a champ but I cannot say the same for modbus RTU. My transmission and recieved lights are blinking on my module so I assume it is broadcasting I just don't believe it is the right data. When I try to connect it does connect but it times out data wise. Any help would be appreciated. Thanks. Attachment serial rtu.JPG (899 Downloads)
universalanalyzers Posts: 7 Rating: (0)
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12/2/2014 10:02 AM	Rate (0)
eisenpferdreiter Posts: 1058 Rating: (93)	universalanalyzers Hello Everyone, I was hoping someone could provide me some guidance. I have attached a picture for how I have set up my system for modbus rtu. I cannot seem to draw the data out of the system. I believe it is an issue with how my DB is setup. I am trying to use the same DB for serial as I am currently using for TCIP. Which is probably a mistake. My modbus over TCIP works like a champ but I cannot say the same for modbus RTU. My transmission and recieved lights are blinking on my module so I assume it is broadcasting I just don't believe it is the right data. When I try to connect it does connect but it times out data wise. Any help would be appreciated. Thanks. You did not state anything about hardware and software used in your configuration and weather you use it as Master or Slave. So have a look here at FAQs for Modbus RTU: Which hardware and software components do you need to establish communication between SIMATIC S7 stations and third-party devices using the MODBUS RTU protocol? How do you establish a MODBUS-RTU communication with STEP 7 (TIA Portal) for the SIMATIC S7-1200? How do you download a loadable driver such as "Modbus Master (RTU)" into a CP341 that is connected to an S7-400 H system by means of an ET 200M? Modbus RTU Communication Module Manual, Modbus master software, Modbus protocol regards
eisenpferdreiter Posts: 1058 Rating: (93)
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12/2/2014 5:53 PM	Rate (0)
universalanalyzers Posts: 7 Rating: (0)	Hello Eisenpferdreiter, I am using TIA portal V12 SP1 I have a S7-1200CPU 1215C DC/DC/Rly and I am using a CM 1241 RS422/485 module. I am running it as a slave or rather attempting to. I am broadcasting the serial signal to a PC which is monitoring data. I have no master block as the PLC is meant to only send data to the PC. I have attached the program. I am beginning to suspect the DB I use for TCIP will not work with RTU but I am not entirely sure. Thanks.
universalanalyzers Posts: 7 Rating: (0)
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12/2/2014 9:41 PM	Rate (0)
William_B Posts: 5225 Rating: (1204)	universalanalyzers Hello Eisenpferdreiter, I am using TIA portal V12 SP1 I have a S7-1200CPU 1215C DC/DC/Rly and I am using a CM 1241 RS422/485 module. I am running it as a slave or rather attempting to. I am broadcasting the serial signal to a PC which is monitoring data. I have no master block as the PLC is meant to only send data to the PC. I have attached the program. I am beginning to suspect the DB I use for TCIP will not work with RTU but I am not entirely sure. Thanks. Hi. Please be careful to not mix Modbus RTU with Modbus TCP/IP. Modbus RTU works over a serial line RS232 or RS485. A desktop PC might have RS232 (DB9 / DB25) port but only special PCs include an RS485 port. In Modbus RTU the MASTER has the "say". The SLAVE listens to what the MASTER asks and responds only to what has been asked. Either the PC or PLC must be the MASTER. Modbus TCP/IP works over the Ethernet connection (PLC Profinet port; PC network port). In this concept there is a SERVER and CLIENT. Master and slave roles are more fluid. So having said all this.. what do you want to do? w @anyone: please join in to help this guy when he responds. thanks.
William_B Posts: 5225 Rating: (1204)
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12/3/2014 4:33 PM	Rate (0)
universalanalyzers Posts: 7 Rating: (0)	Hi William, I previously had modbus tcip setup and it was working great. Then the customer said they wanted a serial connection instead. I have tested the system using two different converters. I have changed the system to RS485. I have used a 485 to ethernet converter. The converter has an ip address so it sort of acts like tcip. I still get timeout messages. Then I decided to try serial only. So I ran a different converter one from 485 to 232 to my computer(have the adapter) still timing out. Yesterday I made sure my computer was working correctly. I set up a second laptop to serve as the slave and my main to serve as master and it worked perfectly. So the issue is with the PLC. Either my data is incorrect in the program or I wiried something up wrong. I followed the pinout of the module which should be 2(+), 5(GND) or 1(GND) I don't know if it matters which ground I use, and 8(-). That is why I was asking if my data block could be setup wrong. As the data block I use works great for tcip but doesn't work for RTU? Doesn't make sense to me why it would work for one but not the other when it is non symbollic. I have tried pointing to the data in the block using a "any pointer" P#DB45.DBX0.0 byte 135 at the recommendation of tech support but that didn't change anything. Thanks for the help! Best regards.
universalanalyzers Posts: 7 Rating: (0)
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This contribution was helpful to 1 thankful Users William_B

4/29/2017 3:12 PM	Rate (0)
Mahdi.R Posts: 1 Rating: (0)	Hi I am experienced with tia portal and have develope several modbus application with that. But there is a problem with broadcating data (address0). When i use this address the modbus_master function waits untill the time out elapsed and this subject increase my network cycle time. Have anyone sample program for this.
Mahdi.R Posts: 1 Rating: (0)	Last edited by: Mahdi.R at: 4/29/2017 3:14:50 PM
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4/30/2017 8:46 AM	Rate (0)
Duris_I Diamond Expert Joined: 4/28/2015 Last visit: 11/27/2025 Posts: 10970 Rating: (843)	Hello, in this case I recommend use different instructions COMM_LOAD with different input parameter RESP_TO filled with suitable value. You need make also your own control with different COMM_LOAD and MASTER instructions to not mix with each other, mean broadcast and respective Slave address sends.
	With best regards... Ivan Duris Flight: OMICO
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4/30/2017 10:00 AM	Rate (0)
#dP Posts: 2826 Rating: (692)	Hello. Very probably you have already code in place to capture any error code that comes up, regardless if it is not held for long. Assuming then there is no error. According to the PLC system manual: S7-1200 Programmable controller System Manual, 07/2011, A5E02486680-04 The value of 0 is reserved for broadcasting a message to all Modbus slaves. Modbus function codes 05, 06, 15 and 16 are the only function codes supported for broadcast. This is a quote from an old manual. So we assume that MB instruction version is not the issue. The only thing left is that the provided MB instruction does not provide internally for system wide broadcast. There should be some time-out instead of the slave's acknowledgement response. Consider putting in place your own time-out. When your broadcast request is done, wait for a short time and then cancel the request. Assume your DONE state. For a test check manually all the slaves if they indeed received the broadcast. When this does not work, get Siemens Support formally involved. Mahdi.R Hi I am experienced with tia portal and have develope several modbus application with that. But there is a problem with broadcating data (address0). When i use this address the modbus_master function waits untill the time out elapsed and this subject increase my network cycle time. Have anyone sample program for this.
#dP Posts: 2826 Rating: (692)
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4/30/2017 9:52 PM	Rate (0)
danw Gold Expert Joined: 3/28/2010 Last visit: 12/30/2025 Posts: 1202 Rating: (220)	What's the problem? The Master and slaves are doing what they're supposed to do. In broadcast mode, with a single instruction the Master writes the same identical data (at the same register address) to all the slaves. No slave replies/responds (by definition). The Master waits for what is typically called a "Turnaround delay" time period to allow the slaves to process the request/instruction. What you describe is what Modbus is supposed to do. So what's the problem? Why are you using a broadcast message?

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