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Modbus RTU - RS485 - Help please

Created by: BrianP at: 1/11/2011 7:49 PM (7 Replies)

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1/11/2011 7:49 PM	Rate (0)
BrianP Posts: 8 Rating: (0)	Hello everyone. Thanks for taking the time to read my question. I would consider myself an intermediate PLC programmer. I've used the Siemens 1200s fairly regularly. Here' is my dilemma. I've created a program that uses a 1200 and a touchscreen. My client now wants to be able to access as many functions from the touch screen as possible using Modbus. I've never used Modbus, but I've researched quite a bit and have actually set up 2 1200s passing DBs to one another using the Modbus Master/Slave blocks in order to get some understanding on how it works. The main problem is that my client will not be using another PLC, but a 3rd party HMI software to monitor and set values. I've set up my PLC to act as a slave, but how do I address everything so that he may write/read values in my PLC? Does this make sense? Thanks again for your time and help.
BrianP Posts: 8 Rating: (0)
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This contribution was helpful to 1 thankful Users rasht

1/12/2011 10:19 AM	Rate (0)
Denilson Pegaia Diamond Expert Joined: 9/23/2005 Last visit: 12/28/2025 Posts: 4434 Rating: (1487)	Hi first, take a look at some tips about modbus in: /tf/WW/en/Posts/34101#top second, take a look at S7-1200 manual, SIMATIC S7 S7-1200 Programmable controller http://support.automation.siemens.com/WW/view/en/36932465 , chapter 6.3.2.3 MB_SLAVE, about MB slave functions codes supported by the S7-1200
	Denilson Pegaia Siemens Industry Sector Technical Support
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1/17/2011 7:56 PM	Rate (0)
Slack Posts: 30 Rating: (0)	If your customer's HMI has an etherent port. I'd stick with Modbus TCP. The ModbusTCP Server that was written for the S7-1200 works quite well. (Uses up some working memory - but fine otherwise). Saves you the cost of the RS-485 comms module - and is much faster.
Slack Posts: 30 Rating: (0)
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1/18/2011 6:09 AM	Rate (0)
BrianP Posts: 8 Rating: (0)	I know, but the client is requesting the use of RS485. I have it hooked up, but I'm not talking correctly, I believe. I'm not getting any error on the Error word on the Slave FB, but I keep getting the DR bit high when I try to poll it from the Master. The NDR bit never goes high either. What does the DR bit signify? There is nothing in the 1200 manual about it. Thank you. -=BrianP
BrianP Posts: 8 Rating: (0)
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1/18/2011 10:20 AM	Rate (0)
Denilson Pegaia Diamond Expert Joined: 9/23/2005 Last visit: 12/28/2025 Posts: 4434 Rating: (1487)	Hi, when a problem is too complex to be solved, I suggest divide it in little parts: Try to mount simple configurations, like suggest open. Use PC and Modbus simulators to read / write values. These simulators allow you see the data trafic. Aftersome sucessful some applications, you will be secury of your knowhow and project and you will be ready to change to the real challenger.
	Denilson Pegaia Siemens Industry Sector Technical Support
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1/18/2011 5:06 PM	Rate (0)
BrianP Posts: 8 Rating: (0)	Ok, thanks for the help. Here's an update. I'm using a Modbus master simulator on a PC. Since it was a PC, I knew it was outputting RS232. So I changed the module to a 232 module and I was able to communicate and change registers. Which is great. But since I need to use RS485, I bought an RS232 to converter and I've put the 485 module back on the PLC. I see the TX and RX lights flash when I poll the slave, but the only output I'm getting for each poll is the DR bit goes high. I don't know what that is. Also, my modbus master is stating that the slave is timing out, even though I've got the timeout response set to 2000ms on the slave and master. Am I doing something wrong here now? -=BP
BrianP Posts: 8 Rating: (0)
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1/31/2011 1:46 PM	Rate (0)
bakerjw Posts: 126 Rating: (18)	DR is data read. Any time that a Modbus Slave accepts a request from a Master and replies with data then DR will go true for one scan. This tells you that the slave is responding to the Master otherwise you would get nothing or the ERROR pin would go true for one scan with a valid error message.
bakerjw Posts: 126 Rating: (18)
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1/31/2011 7:01 PM	Rate (1)
0_Einstein Grandmaster-Expert Joined: 7/7/2010 Last visit: 12/23/2025 Posts: 16325 Rating: (2520)	BrianP Ok, thanks for the help. Here's an update. I'm using a Modbus master simulator on a PC. Since it was a PC, I knew it was outputting RS232. So I changed the module to a 232 module and I was able to communicate and change registers. Which is great. But since I need to use RS485, I bought an RS232 to converter and I've put the 485 module back on the PLC. I see the TX and RX lights flash when I poll the slave, but the only output I'm getting for each poll is the DR bit goes high. I don't know what that is. Also, my modbus master is stating that the slave is timing out, even though I've got the timeout response set to 2000ms on the slave and master. Am I doing something wrong here now? -=BP My advice to you is to contact Siemens tech support right away. Ask to speak to their S7-1200 RS485 specialist. They can explain how the 485 module configuration options best apply to your specific application. You will get the answer faster than can be explained online. The answer should include things like how a request appears, what starts it, what stops it, what baud/parity/stop bit you need, how long is the shortest and longest message the PLC will receive. These settings are essential for the Receive PtP instruction to function properly. Also, you cannot put the value into a string easily, but you can put the message directly into a stringby following the instructions in the 485 receive FAQ for the S7-1200. It came out last quarter of 2010. It really is helpful.
	science guy
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This contribution was helpful to 1 thankful Users grrrrrroooo

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