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MODBUS RTU with CP341

Created by: TheAtomicGuy at: 4/7/2009 9:05 AM (5 Replies)

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4/7/2009 9:05 AM	Rate (0)
TheAtomicGuy Posts: 5 Rating: (0)	Hi, My question concerns connection via ModbusRTU Protocolbetween CP341 as Master and digital protection device (CZAZ-UM) as Slave. I'm not asking you for the solution but just for the way I should start solving this subject, as far it is my first step in Simatic S7. -- Best Regards, TheAtomicGuy
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4/7/2009 10:54 AM

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dchartier

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Joined: 9/27/2006

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Hello AG;
Let's say you have a S7-300 CPU and a CP 341 (with appropriate interface for RS-232 or RS-485), and your basic project is started, at leastin Step 7's HWConfig editor.
First you will need to purchase the loadable driver Modbus-RTU master (I am presuming it is RTU, but there is also a driver for Modbus ASCII if required). Here is the information available on the A&D Mall on this driver:

6ES7870-1AA01-0YA0

SIMATIC S7, MODBUS MASTER V3.1 SINGLE LICENSE F.1 INSTALLATION R-SW, SW AND DOCU. ON CD, HW-DONGLE, CLASS A, 3 LANGUAGES (G,E,F), EXECUTABLE U. STEP7 FROM V4.2, REFERENCE-HW: CP341 AND CP441-2

This package will come with a small hardware dongle that has to be loaded in the back of the CP341 in order to enable the Modbus RTU master functions. It also comes with the programming blocks (and program samples) required for this application and a parameter assignment software that allows you to setup the CP 341 as a Modbus master in the HWConfig editor.

You will start by setting up a PtP connection in your project, between the CP 341 (already installed in HWConfig) and your serial partner. The connection ID that Netpro will attribute will be refered to in the program blocks you will use later.
Next you will set the PtP parameters for the CP 341 as Modbus master for the project (in HWConfig again): baudrate, parity...
Next you will use the program blocks loaded in the Step 7 libraries when you installed the loadable driver earlier. These are described in the manual for Modbus RTUmaster, along with an example project; see chapter 8 of the following link:

Manual:” Loadble driver, Modbus RTU, CP341 master”

http://support.automation.siemens.com/WW/view/en/1220184
You will find details of all the steps I am pointing out in this document.

If you follow the example, you can build a basic application easily enough, even though the example is in STL. You will be requested for the connection ID provided earlier when you want to setup the send and recieve blocks. What you alsohave to remember is that all information for a Modbus job must be written to the first four variables in the SEND source DB; this is DB 42 in the example program. Here is where you set the slave address of your partner, the Modbus function code, the start register address and the number of registers to be sent to the slave. So if you connevt to multiple slaves, or request more than 1 job for a single slave, then you have to build a new, distinct Send DB for each request. The Recieve DB must also be setup appropriately, of course, and a new one provided for every Modbus job.

One very important point you will have to pay attention to: cabling must be correct between master and slave(s). As long as the CP 341 does not see a "correct" electrical connection on the pins it must use for connection, the TX line will not flash. But the exchange lines must be absolutely perfect before the RX line starts blinking, showing that the slave is replying to the master. There is not much more diagnostics on the connections, I am afraid. And it can be troublesome. Study the example diagrams that you find in the appendices of the suggested manual, and the documentation from your slave's manufacturer. I have lost many hours of work trying to figure out what was wrong at that level...

Hope this will help and is not too confusing,
Daniel Chartier

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4/7/2009 11:39 AM	Rate (0)
TheAtomicGuy Posts: 5 Rating: (0)	Hello Daniel, Your answer is very helpful. However, my project consist of two S7400H (redundant) connected via PROFIBUS with CP 341 and serial partner which is connected via MODBUS with CP 341. The idea is that I only have to read data from serial partner. Any data is going to be send to digital protection device. As you can see the subject seems to be quiet tough :-) -- Best regards, TheAtomicGuy
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4/7/2009 12:47 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 3/14/2025 Posts: 12344 Rating: (2707)	Hello AG; Please refer to the manual I suggested earlier, chapter 5 describes the different Modbus functions available on the CP 341. You will notice in the description of every function a Request message (from the master to the slave) and a Reply Mesgae (from the slave to the master). Anytime you need data from the slave you must send a request from the master, telling the slave that you are needing data from this slave (slave address), the Modbus function you will use (indicating the type and location of the requested data), the startiong address of the requested data in the slave's memory mapping and the amount of registers to be sent back to the master. Modbus uses Modicon addressing (it was designed for that PLC 30 years ago), so registers will have addresses starting at 00000, 10000, 30000,, and 40000 (see your slaves Modbus mapping). Modbus functions are assoxciated to a single type of register. So, say you need data that your slave's Modbus mapping shows at addresses 40032-40048, you will need to set, in the master's Modbus job (in the Send DB); -SLAVE ADDRESS: 3 (for example, based on the slave's configuration) -MODBUS FUNCTION: 3 (function 3 requests mutliple output registers, starting with 40000) -START ADDRESS: 32 (points to 40032) -LENGTH: 16 (end address of the requested registers is40048) Every time you want the slave to send yiou these registers, you will need to SEND these job settings using the program blocks shown in the manual's example. And your recieve DB will have to be formatted to hold the full amount of registers required by the Modbus job. Please study the manual I suggested earlier so you understand the requirements of Modbus communications in general; then you can look into using a CP 341 to setup the comm link. Hope this helps, Daniel Chartier

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4/7/2009 2:56 PM	Rate (0)
TheAtomicGuy Posts: 5 Rating: (0)	Hi Daniel, Now I'm studying the manual concerning Modbus Master communication - as you suggested. I think it will take a while to understand it. But don't worry, sooner or later I'm sure I'll have some question again. -- Best Regards, TheAtomicGuy
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9/29/2009 5:46 PM	Rate (0)
Nunya Posts: 3 Rating: (0)	This package will come with a small hardware dongle that has to be loaded in the back of the CP341 in order to enable the Modbus RTU master functions Siemens help desk told me the dongle was needed for master and slave devices. Beware FC7 and FC8 , some versions do not work with Modbus RTU. The ones on the CD examples seem to work though.
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MODBUS RTU with CP341

Created by: TheAtomicGuy at: 4/7/2009 9:05 AM (5 Replies)

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MODBUS RTU with CP341

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