Modbus RTU, Master Slave, not communicating

I'm a licenced electrician and I took courses in Network administration.

I have problems getting two modbus devices talking to each other over a 2 wire 485 modbus RTU.

one is a twido plc that has an ethernet port, a din port and a RS485 3terminal adapter.

the other is a Delta Controls DSM BacNet Gateway, with a 485 port flashed with modbus protocol.

What Ive learned so far.

the Gateway has to be grounded for the controller to communicate with my plc.

My port is setup as a slave 19200br, 8N1 RTU which is the same shown on his controller.

He is getting invalid response from his port statistics. I've talked to both his tech support and Schneider's and all say that their end is well configured....as usual.

at this point I'm leaning towards intalling a 485 to ethernet converter and hopefully this will rectify my comm problems.


My question is do these converters usually retify these types of problems or am I simply wasting my money?

How about a OPC Server? its a bit overkill for a small system. But I'm not too familiar with opc in an automation setting.

Are there any issues with communicating with rs485 that I should be aware of? I'm reading up on as many different angles as I can get my hands on but still not knowing why.

If you are absolutely sure you have the same setup at bot ends (except for 'master' and 'slave') then try swapping the 2 leads of the RS485 connection.

Thanks Bernie,

I swapped the wires a few times using different baud rates.

His gateway controller has to be a master so my plc port is set to slave.

I'm not 100% sure that his Byte Format is setup right.

I read up on his documentation and it was saying that its setup is:
8 data bits, no parity and 1 stop bit with CRC checking

Now I can modify these settings on my side, which is what I did. But on his side he is only able to modify the baud rate. So this is why I'm assuming that 8N1 is the format.

jechlu said:

over a 2 wire 485 modbus RTU.

Click to expand...

Although it is called 2-wire, RS485, you generally need three wires to make it work.

You may want to read up on the RS485 tech notes that B&B Electric has on their sight:
http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp

thanks reading up now....

In my opinion, every serial connection diagnostic effort should begin with a data analyzer or capture of some kind. Modbus is particularly simple to decode, so even a freeware serial terminal program can do the job.

But you have to be able to connect to the RS485 physical network, and that's a connection most PCs don't have.

A B&B RS232/485 converter often does the job well.

I am fortunate to have Frontline Test Equipment's newest ComProbe II RS485/USB interface, which puts custom firmware into the VersaTap hardware from Stratus Engineering.

For RS-232 you can often get by with a simple splitter or "Y" cable. For RS485 there's no way to avoid tooling up to the tune of $400-$500.

If you are casually involved in serial communications, the Stratus Engineering devices and Docklight software are a very good value.

If you do industrial networking for a living, the FrontLine Test Equipment suite is indispensable.

Thanks Ken,
I'm currently working for a company at the moment but slowly trying to get into my own. So I'll be closely reading up on Frontline.