Communication PLC with Modbus

Hi bernie_carlton, need some help, I need to read the value of a sensor with Rockwell MicroLogix 1100 PLC with Modbus RTU, but not'm getting!

You don't need to address a post to one member. Many are capable of helping.

Please provide details concerning the device from which you will be reading information (preferably a link), details of your wiring, and a zipped post of your .RSS file showing your attempt so far.

I am starting the project, eg, need to do a reading with a MicroLogix 1100 PLC with Modbus network, need to get a value from a sensor or something to give a command to the PLC, but not sure how to do this in Modbus RTU. It would be possible to have an idea?

The Micrologix 1100 Instruction Set Reference manual has information in Appendix D on setting up the RS485 network. In section 21 it has information about the message (MSG) instructions.

Once you have chosen your Modbus device and have made the first steps then post back with the information I requested noting any point where you have problems.

Okay, Thankyou bernie_carlton!!

Note - the 1763-NC01 is the recommended and easiest way to break-out the RS485 wiring from the 8 pin mini-din connector on the Micrologix 1100 to individual wiring terminals.

bernie_carlton said:

Many are capable of helping.

Click to expand...

Just not as good as you

geniusintraining said:

Just not as good as you

Click to expand...

Truth!!!

Has anyone tried to connect a ML1400 to a Pneumercator TMS Panel? I am currently trying to pull data via Modbus from the TMS panel to the ML1400. I have followed the many examples of Comm 0 setup shown on this site. I have verified all of the settings at both ends and wiring. However, when I run the code I get error 37, "Message timed out in local processor". I am wondering if I just wired the dang thing wrong, but its modbus... not many ways of doing it wrong.

I am using Channel 0, Modbus RTU Master, No Handshaking (485 Network)
In the MSG Setup: Channel 0, Command 03, Table N10:0, 1 Element, 16bit, Time out 3, Address (used values per TMS Spec), Slave Node 1 (verified at TMS).
I am using the 1763-NC01. The TMS only has three landings, A+, B-, and Shield which I have wired to the 1763-NC01 A, B, Shld.

I have tried changing nodes, different wires, switching around connections, checking the termination resistor. No matter what I do, I get the same response from the PLC: "Message timed out.". I even have two identical PLCs and TMS panels to work with just in case there was an out of the box failure. No difference in behavior.

Thanks to anyone that can help or shed light on some stupid mistake I made.

Welcome to the Forum !

Try connecting the Data Common "COM" terminal on the 1763-NC01 to the shield or to chassis ground at the TMS system.

RS-485 always requires a common reference against which Data A and Data B can be measured.

Rockwell Automation has always provided that as an isolated Data Common conductor, but plenty of other vendors go the cheap route and use the shield or the chassis ground or even Earth ground as the common reference point.

have you verified that you can talk to the device directly with a software like modscan or modbus poll using a 232 to 485 converter?

Thanks for the help

So I finally got it working.

First, the recommendation to bond the common and shield ground seemed to help. I also found an interest quirk to the TMS panel. In my mind, when the terminator switch to turn on the end-of-line resistor for the modbus says "on" and "off", I think that on means the resistor is applied. Turns out I was wrong, but that isn't written in the specs.

So, hours of lost time for switch. lol.

Thanks everyone.