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CP341 Modbus Master function code 23 (17hex) (Read/Write 4x registers)

Created by: timoreardon at: 6/1/2010 3:45 PM (2 Replies)

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6/1/2010 3:45 PM	Rate (0)
timoreardon Advanced Member Joined: 5/19/2010 Last visit: 2/7/2024 Posts: 22 Rating: (2)	Hi, Can someone point me in the right direction and clarify something for me. I have a CP341 (6ES7 341-1CH02-0AE0) RS485 communications module and have purchased the Modbus master licence and dongle to go with it to make it a modbus master. The modbus slaves I am hoping to communicate to only support one function code of the Modbus protocol, namely function code 23 dec (17hex) which is Read/Write 4X registers. All other function codes are ignored. According to the "S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is master" manual, on page 40 a table shows these function codes are supported: 01(dec) Read Coil Status 02(dec) Read Input Status 03(dec) Read Holding Registers 04(dec) Read Input Registers 05(dec) Force Single Coil 06(dec) Preset Single Register 07(dec) Read Exception Status 08(dec) Loop Back Test 11(dec) Fetch Communications Event Counter 12(dec) Fetch Communications Event Log 15(dec) Force Multiple Coils 16(dec) Preset Multiple Registers where as the latestModbus protocol specification document I have found on the www.modbus.org website shows that there are several more function codes in the protocol. My question is, does the CP341 with Modbus master support function code 23 dec(17hex)? If it does not support this function code, why does it not support the whole of the modbus protocol? It could be a very expensive problem if it does not support this code!!!! Thanks for any information that anyone can supply. Regards Tim

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6/1/2010 4:23 PM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 11/30/2024 Posts: 3028 Rating: (1057)	Hello timoreardon I'm afraid to say that I don't have good news for you as Function code 23 is definetly NOT supported by the CP341 Modbus Master. I must also say that I've never come across aModbus Slave which does NOT at least support FC 3 and 16. FC 3 and 16belong to ModbusConformance Class 0 (the lowest "common denominator" Modbus Conformace Class, this is the minimum useful set of functions, for both a Master anda Slave). Conformance Class 1 requires support for FC1, 2, 4, 5, 6 & 7. Conformance Class 2 covers 15, 20, 21, 22, 23 & 24 and typically found in HMI systems. As you see, the CP341 Modbus Master does fully cover Conformance Class 0 & 1 and a bit of CC2, so the question really is: Why does your Slave only support FC23? I'd put the hard word on the supplier of it and ask them to add at least CC0 compliance to it (i.e. FC3 & 16 support). Good luck and I hope this helps at least somewaht
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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6/3/2010 11:52 AM	Rate (0)
timoreardon Advanced Member Joined: 5/19/2010 Last visit: 2/7/2024 Posts: 22 Rating: (2)	Hi Fritz, Thanks very much for the reply. It's unfortunate that the Modbus master protocol doesn't support the full modbus protocol, hopefully Siemens will accept the return of the module and modbus driver. I contacted the supplier of the modbus slave and explained the problem. Their answer was that all modbus masters should really support the full modbus protocol and said it was simpler in their opinion to be able to read and write paramters to and from their device using just the one function code command. I can see their point, but also can see why Siemens haven't included all the functionality as it just makes it simpler. It's just a shame I only found this out in the last few days. Hopefully Siemens will upgrade their software to include all the modbus function codes in the future so others don't fall into the same trap. Plan B is to use a Profibus DP to Modbus gateway from a different manufacturer all together, fingers crossed!!! Thanks again for your reply Fritz, saved mea few more hours of playing with something that wouldn't work. Cheers Tim

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