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Question: how to program comm. over rs-485

Created by: FransWijnen at: 1/7/2010 3:24 PM (5 Replies)

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1/7/2010 3:24 PM	Rate (0)
FransWijnen Posts: 3 Rating: (0)	Hi everybody, I am new to S7 basic v10.5, did a short course on basic programming, which was enough for what I wanted to achieve so far. However, I now face an unknown for me and that is how to communicate over rs-485 with another instrument, in this case a vacuum pump controller. The pump controller has an rs-485 interface, and I could/should buy comm. modules for the S7-1200 to establish an rs-485 link between the PLC and the pump controller. But how does one program data exchange then? I have done such programming before, but not in S7, and I think on a bit higher level. If anyone has a simple example program that I could look into that would be highly appreciated. Best regards, Frans
FransWijnen Posts: 3 Rating: (0)
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1/7/2010 6:26 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	RS485 is the the electrical standard (the physical medium if you like). Undoubtably the communications will be based on some software protocol (e.g. Profibus, Modbus etc.). As an analogy, think of RS485 as the telephone line. The telephone line just provides the means of communications. The language you use of the phone would be the protocol (with English, German, French etc being analogous to Profibus, Modbus etc.). To take the analogy further, the conversaion you have in the selected language would be analogous to the data being transferred. So you would need to find out what protocol the vacuum pump controller is using. If the protocol is an industry standard, then you would simply need to get a suitable interface CP/CPU to setup communications. For example, Siemens provide Modbus specific CP's for interfacing with modbus devices. If the protocol is not an industry standard, you would need to write you own protocol driver and communicate over a standard CP or find some protocol convertor to convert the vacuum pump controller protocol into an industrial standard.
	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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1/8/2010 2:01 PM	Rate (0)
FransWijnen Posts: 3 Rating: (0)	Dear smiffy, Thanks for the intro, it's clear, but I do not see how this helps me. Maybe I was not clear in my question. So, let me try to clarify. Then let's now talk about the S7-1200 - which is the PLC I'm using. It has comm. modules for RS-232 and RS-485. I don't have one yet, but I do have S7 basic v10.5 SP1, which offers me send/receive commands for the RS-485 bus. Because I am unfamiliar with the input/output of those block (I was expecting a text string of an array of hex number (characters) that I could input/output over the RS-485), I do not understand how to use these. So the question is: * where can I find some more documentation on these functions (except in the help) * are there any examples available, which could clarify these functions - I work a lot with labview, which offers examples in its help - certainly, where no standard functions are concerned, that is very helpful. best regards, Frans
FransWijnen Posts: 3 Rating: (0)
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1/8/2010 4:52 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	You keep asking how do I communicate over RS485. But as I have already said that is not really the right question to be asking. It is very easy to communicate over RS485 (or any serial interface for that matter), but you have to know what protocol to use. You can easily send / receive ASCII data over RS485 by using any of the serial CP's (e.g. CP341, CP441). Firstly you would have to parameterise the CP to use ASCII point-to-point interface. To do this you would have to load in the point-to-point parameterisation software. It is available to download at:- PTP Parameterisation software Once you have set the CP to point-to-point ASCII communications you would use the correct SFC/SFB for the CP you are using to send / receive data. The CP341 uses P_SND_RK and P_RCV_RK to send and receive data respectively. The CP441 uses BSEND and BRECV to send and receive data respectively. However, I think just sending ASCII data will not do the job for you. The vacuum pump will expect to see or give out data in a specific format (this is the protocol that I speak of). How does the vacuum pump know when to send it's data? Does it do it cyclically or in response to some for of request? Again these questions are catered for in the protocol. Do you have any documentation on the vacuum pump controller?
	Last edited by: smiffy at: 1/8/2010 4:53 PM 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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1/11/2010 12:50 PM	Rate (0)
Robster Gold Member Joined: 5/28/2008 Last visit: 3/6/2025 Posts: 1343 Rating: (141)	Hi, I agree with smiffy that pump maybe has a protocol like Modbus or USS or anyting else. Then it would be easier to handle because of an existing library. But appart from that, you can handle communication via RS485 Communciation Modul by the operations SEND_PTP and RCV_PTP. For configurating the port, you can use hardware configuration and enter there fixed parameters like baud rate, parity and so on. There you can also parametrize conditions for sending and receiving messages. If you like changing these configurations during runtime, there are the operations PORT_CFG, SEND_CFG and RCV_CFG. For sending and receiving messages you need a buffer, where your data are located. The order of data in the buffer must according to protocol whatever your pump except. Mostly it has a function code, some starting address, the relevant data and maybe checksum. The answer from pump is in the same order and has to be analyzed. The datatyp is not important for sending and receiving. Here is an application Example , that can helps you. There is RS232 Modules used, but the functionality is the same like RS485. [url]http://support.automation.siemens.com/WW/view/en/38471716[/url] regards

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1/11/2010 1:25 PM	Rate (0)
FransWijnen Posts: 3 Rating: (0)	Hi Robster, Thank you for your answer... I was starting to worry that once more I did not succeed in explaining my problem, but you picked it up the right way.@ Smiffy: you're absolutely right, I need to implement a protocol, but my problem was myunfamiliarity with the basic comm. functions in S7. Anyway, the example clarifies a lot on how and where to store the buffer and things like that. I think that with this example I'll manage to get something working. Thanks again! Frans
FransWijnen Posts: 3 Rating: (0)
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