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CP340 RS485 & Modbus

Created by: Brad at: 8/1/2007 8:56 PM (10 Replies)

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8/1/2007 8:56 PM	Rate (0)
Brad Posts: 5 Rating: (0)	I have existing PLCs which used CP340 to talk Modbus RTU (so I'm told) via RS485 to a converter to RS232 and into Foxboro DCS. I've been asked to migrate those to a direct RS485 link to a newer DCS module with a Modbus RTU protocol. I'm able to transmit from the DCS and I see a receive at the CP340, but nothing comes back. I'm hearing from some that the CP340 is not talking Modbus, although I see the old DCS module image is, indeed, Modbus. Any ideas on troubleshooting this?
Brad Posts: 5 Rating: (0)
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8/2/2007 8:55 AM	Rate (0)
Wizard Diamond Expert Joined: 4/24/2006 Last visit: 9/14/2023 Posts: 8592 Rating: (1132)	Hi Brad, When you are using a CP340, you cann't use the modbus driver. Only when you have CP341 it is possible to use a Modbus driver. Please have a look at following link: http://support.automation.siemens.com/WW/view/en/12964856 Best regards, Wizard

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8/2/2007 3:30 PM	Rate (0)
Brad Posts: 5 Rating: (0)	That's interesting that you say that ... and I see that the CP340 supports only 3964(R), ASCII and printer protocols ... and yet the current communication is definitely tagged as a Modbus image in out DCS gateway. I see from the datasheet that the CPU312 has an integralRS485 interface ... but I don't see anything about supported protocols for that. Is that CPU interface only used for programming the CPU?
Brad Posts: 5 Rating: (0)
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8/2/2007 3:36 PM	Rate (0)
Wizard Diamond Expert Joined: 4/24/2006 Last visit: 9/14/2023 Posts: 8592 Rating: (1132)	Hi, The RS485 port of CPU312 is only suitable for MPI protocol. Best regards, Wizard

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8/2/2007 4:19 PM	Rate (0)
Brad Posts: 5 Rating: (0)	I'm really stumped here. The current setup (which has been running trouble-free for about 10 yrs) is using 3964R protocol in our DCS module (although the module image says it is Modbus). Perhaps our developers modifiedan existingModbus driver to create the 3964R driver and didn't change the image code? That was back in 1995 and the guy who wrote the interface isn't around anymore. With our upgraded foreign device interface, it appears we only have 3 drivers - Modbus, OPC and AB ControlLogix. Oh well ...
Brad Posts: 5 Rating: (0)
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8/6/2007 9:11 AM	Rate (0)
Irmantas Bronze Member Joined: 3/7/2006 Last visit: 4/26/2023 Posts: 42 Rating: (7)	Hi, you can use for modbus protocol all siemens PTP port (340, 341), but you must self write modbus protocol comunications. I wrote modbus protocol (slave) for cpu314-ptp and it working. If you want use siemens modbus program you can use only 341 or profibus-modbus converter http://www.t-h.de/index/Deutsch/89115

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8/8/2007 9:15 PM	Rate (0)
Brad Posts: 5 Rating: (0)	I do not have the Siemens S7 configuration software, but I would like to discover the address in a PLC. Is there a command I can send to the PLC that will force it to reveal its address? I have a laptop Modbus simulator which I can use.
Brad Posts: 5 Rating: (0)
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8/12/2007 12:57 PM	Rate (0)
Late Platinum Expert Joined: 1/7/2006 Last visit: 7/3/2023 Posts: 2705 Rating: (314)	Hi Brad, You need in the CPU or CP340, CP341 a program that haves the Modbus Protocol implemented, when somebody else have made that, then you have to ask which Modbus-address is implemented. If you have the STEP7 code and not STEP7 by yourself, I could look inside which address is used, you may send your email-address in a private message.
	I hope that the answer will help, if you have still questions, don't hesitate to ask me. All the best, Late Never say never, think in solutions, not in trouble......
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9/10/2007 10:19 PM	Rate (0)
Brad Posts: 5 Rating: (0)	We were finally able to establish communication with the S7, but we had to use one of our older interfaces. Here is what we found. Perhaps someone can tell me if this is normal behaviour for the Siemens S7. In our particular case, holding registers 768-783 are used. Ifwe poll anunused register (for example a Function Code 3 to address 40001), the PLC stops running. It should reply with an exceptioncode 83 to say that register 400001 is invalid. Our developers had never seen this with other PLCs and have started testing for it. Has anyone ever seen this before with Modbus on a Siemens S7 ?
Brad Posts: 5 Rating: (0)
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9/13/2007 11:18 AM	Rate (0)
dixinke Member Joined: 9/13/2007 Last visit: 8/29/2022 Posts: 15 Rating: (0)	Code 83 mean sth error with the register area, your command is a standard Modbus protocol command, but the third party device do not open all the register area for you. You can try like this, only read one data as 01 03 00 00 00 01 CRC. (If you modbus slave address is 1)

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