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Make my own Modbus RTU library for S7-1200

Created by: Radastan at: 3/14/2013 9:21 AM (6 Replies)

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3/14/2013 9:21 AM	Rate (0)
Radastan Posts: 56 Rating: (8)	I have a problem with Modbus RTU library of S7-1200, I can't swap CRC bytes. The only solution to solve it is make my own Modbus Master RTU library, ¿is possible? ¿any help?
Radastan Posts: 56 Rating: (8)
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3/14/2013 11:34 AM	Rate (0)
TIBI68 Gold Member Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating: (147)	Dear Radastan , why do you need to swap the CRC bytes? I do not see any reason for it can you explain? If you would be able to change it it is not following the modbus RTU standard. Unless you need to communicate to a devicefollowing its own protocolandclosetothe modbus.
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3/14/2013 12:00 PM	Rate (0)
Radastan Posts: 56 Rating: (8)	TIBI68 why do you need to swap the CRC bytes? I do not see any reason for it can you explain? If you would be able to change it it is not following the modbus RTU standard. Yes, it is no following modbus RTU standard, it swap CRC bytes, but the rest of the message is modbus standar. I search for information and see some modbus slaves with the same problem, it is usual. On S7-300 I can make my own modifications and swap CRC bytes, but in S7-1200 I can´t see options. You can see how to program a Modbus Master RTU in S7-300 here: http://read.pudn.com/downloads139/doc/599253/S7-300%20as%20Modbus%20Host.pdf
Radastan Posts: 56 Rating: (8)
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3/14/2013 12:10 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	Modbus is an industry standard protocol and have changed very little since its introduction in 1979. It has always encoded its data in high-byte low-byte order (including the CRC). If the device you are interfacing to claims to be modbus then it should work correctly. If it does not, and the CRC bytes are genuinely reversed, then the device is not conforming to the modbus specification and it is therefore at fault. What is the device you are trying to interface with? Do you have a part number for it? Have you contacted the device manufacturer and told them about the issue? If they are selling it as a modbus device, it should be validated as conforming to the modbus specification. There should be no reason to need to program your own modbus solution. However, in case you really want to persue this avenue, then please see my comments in this thread. It will take some considerable effort to make a robust modbus driver though. Your call.
	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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3/14/2013 12:38 PM	Rate (0)
Radastan Posts: 56 Rating: (8)	The slave is a Keller Serie 30: http://www.keller-druck.es/contenidos/pdf/engl/comm_protocol_e.pdf If I try to communicate with this slave I obtain8383 error code (data out of range). I try some ranges without success. In the same network I communicate without problem with the rest of salves, only have problem with Keller one's. Keller say me it's CRC problem. EDIT: SOLVED, Range is not start on 0, start on 1. Address 2 is 3 in decimal, all works fine now. Keller Spain have no idea about his product. :(
Radastan Posts: 56 Rating: (8)	Last edited by: Radastan at: 3/14/2013 1:06 PM
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This contribution was helpful to 3 thankful Users smiffy Kaulquappe TIBI68

3/14/2013 2:51 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 3/10/2025 Posts: 1557 Rating: (540)	Well I never. Having checked the modbus specifications on modbus.org, the CRC is sent in low-byte high-byte order not high-byte low-byte order as per the rest of the data fields. So the Keller manual has it correct. But I find it hard to believe that Siemens has got it wrong with their S7-1200 driver. Otherwise, there would be a massive number of complaints (and a lot of egg on face for Siemens). It has to be fault with the Keller software. Could you send me the data packets for both the request and response packets and I will check them for correctness. (sorry posted this before I saw your edit to say you have it working).
	Last edited by: smiffy at: 3/14/2013 2:53 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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2/18/2020 10:02 AM	Rate (2)
mmr_bonArea Posts: 2 Rating: (2)	Here you are a function for compute Modbus CRC: [snip] Hope that will help you. Attachment Function.txt (432 Downloads)
mmr_bonArea Posts: 2 Rating: (2)	Last edited by: Jen_Moderator at: 02/18/2020 13:56:46 Added function as a txt file.
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