I have Advantech EKI-1521 Gateway module. I need to connect it to S7-1500 CPU over Ethernet port. On other side of EKI module is Modbus RTU device.

How do I access Modbus RTU device registers over EKI module? Is it Modbus TCP/IP server for S7-1500 side? Or do I need to use TSEND and TRECV functions in order to establish communication?

BR,

Milos

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Split from MODBUS COMMUNICATION THROUGH MODBUS GATE WAY MODULE.

Duris_I

Hello Milos,

really don't know exactly that Gateway but if it's specification is on one side Modbus TCP and on another side Modbus RTU, yes. For S7-1500 is Gateway like Modbus TCP Server. You can then with Kaulquappe's infos access RTU Slaves. Modbus general specification (look to TIA about MB FC and respected Siemens MODE), you can set without problems required datas as with RTU but via MB TCP system, you need set different MB_Unit_ID (Slave RTU address) as Kaulquappe sugested.

Update: in some cases (depended to gateway manufacturer) you need also configure Gateway internally with some other settings but you need check this in your Gateway documentation.

Update2: check again carefully that Gateway, it looks to me like general Serial to Ethernet Server. In other situations you need have exactly specified protocols supported by HW!

 Dear Duris,

I believe that this device is not Modbus TCP to Modbus RTU converter. I tried to connect Modbus RTU device on serial side of gateway, and Modbus TCP client on Ethernet side. Using sniffing tool for serial port i could see all packets. This gateway just transfers Modbus TCP frame to serial port, and because frame structure is not the same Modbus RTU device does not reply.

Using Modbus poll application I managed to read data from serial device. In this app I chose Modbus RTU over TCP/IP!!!

What would you suggest me to do in order to establish communication with S7-1500 CPU?

BR,

Milos

Dear Dunis,

I managed to establish communication using TSEND_C and TRCV_C. I made Modbus RTU frame and function for calculating CRC. I packed that frame into TCP frame and I received response from device which was correct. Everything works perfectly :-)

Thanks for helping!

BR, 

Milos

 Hello Milos,

 Could you give me some details how you did receive the data and how you had read the RTU data through TCP frame. I am in the same position as your trying to receive the Power meter data through a RTU to TCP gateway. I am new into this and couldn't get it going. I had followed something Duris suggested, but still at some point i am not clear. If you can help with some photos of documents to get this done, it will be a great help.

Thanks,

jay

Milos Petrovic

Dear Dunis,

I managed to establish communication using TSEND_C and TRCV_C. I made Modbus RTU frame and function for calculating CRC. I packed that frame into TCP frame and I received response from device which was correct. Everything works perfectly :-)

Thanks for helping!

BR, 

Milos

Hello again!

I was using blocks TSEND_C and TRCV_C, not MB_CLIENT. Those blocks have parameter DATA, this is location of data to be sent or where to receive data. Please see attached pictures and everything will be clear.

You receive data in MyDB.MyRcv array. Here you must identify RTU frame and extract data from it. RTU frame should look like this for example: https://www.picotech.com/library/oscilloscopes/modbus-serial-protocol-decoding

If you are creating requests toward serial device one important thing is to, inside RTU frame, create CRC (cyclic redundancy check) bytes. If these are not correct serial device will not reply to you.

If you have any other questions please contact me.

Milos