I'm using RS-485 multidrop. I have 5 devices connect with modbus RTU. If one of my devices down then it will affect other device that cannot get any data. i using MSG function in micrologix for poll data. Any solution regarding this issue? Please Help.

Please help.

 

No problem at all,

The logic works like this:

You probably have ladder rungs that fire off sequentially (or maybe not) but in any case, your program controls when it wants to get data from a remote. Just look for an error condition, start a timer for X seconds and block that rung for a "time-out" period if an error is detected. I would suggest a minute or more.

Everything else that is working will continue at full speed until the "error time out" expires and then the "failed" remote will be retried. If it works, great, it does not, then block communication attempts for another period of time. If you are using MSG instructions, then you probably will need to reduce the "time out" period to get satisfactory performance. If a Modbus device is going to respond, it will do so fairly quickly and the default message timeout is fairly long. I suggest that you shorten it to 1 to 2 seconds.

FYI, there are products that you can purchase that do this automatically and take care of all the messaging and writing of the data to PLC registers. We sell one and if you are interested, contact me directly at jmiille at miille.com.

Good luck!