Basics of MODBUS communication via RS485

[kilca]

Hello again,

 

since your answers are very helpful and I could not find information I need on this forum, I`m starting new topic.

 

I need to set up communication between V280 and multimeter DMK 22 by Lovato.

Here is the tutorial they sent me: https://www.sendspace.com/file/trigbh(file is too big to be attached here on forum)

 

I have several problems with understanding the whole communication. I have working MODBUS IP communication between PLC-to-PLC, where the communication is done through MODBUS R/W Mix, which I understand how it`s done. Here I need to get from the slave multimeter 3 values.

First question: How do I send message to slave? The message should looke like this: 08 04 04 00 00 00 64 63 6A, do I have to send it as vector of HEX MIs? And the message I get back from slave will be vector too?

If you don`t mind to explain this a little bit detailed including a little tutorial how to set it up correctly in VisiLogic, I`d be very happy. I have to solve this in 2 days.

 

Thanks in advance a lot! Unitronics community is great and I will surely write it down to my thesis and recommend you.

 

Best regards,

Martin P.

[Joe Tauser]

I think we missed your window.  I've been busy and the Unitronics office is getting a new computer system, which is why they haven't been answering posts.

 

You'll need to set up one of the serial ports and make a cable to talk to this meter.  Configure another Modbus port as an RTU master.

 

I looked at the documentation.  They did the usual silliness and imply that the user somehow has to generate his own hex codes to implement Modbus.  No, you do not have to do this.

 

Page 10 shows the Modbus Register Address Table (in hex- convert these to decimal).  Note that these are Long registers - I'm a little rusty on getting these into Unitronics but it can be done.  I think you set ML's as the register type in the Read Holding Registers block.

 

Hook it up and start beating on it.

 

Joe T.

[Gerardo Gamez]

Best Regards,

1.             I am having a problem connecting a V350 unitronics by modbus RTU RS485 as slave to a master (modbusPoll software-PC) RS485 4 Wire.

                I have checksum error in this configuration.

                In any other configuration the communication works fine. By other configuration I mean having modbusPoll as master RS485 2 Wire and RS232.

                I am just having problem with mobusPoll as master RS485 4 WIRE.

               Thank you for your answer.

 

2.            I see by default RS485 2 WIRE on V350 PLC unitronics. Is there any model within the catalog of unitronics, having RS485 4 WIRE available (physically).?

               Thanks for your answer.