First all , I am new Here and just want to say "HI" to everyone. I am new to the Allen Bradley world but Have been around PLC programming for over 5 years. My question is the following: I have a AB micrologix 1400 series B controller. I have added the 485 module, I need to pull back some values from a instrument that is setup as a 485 slave device. I have went into the Channel 0 setup and have setup that part. Where Do I setup a modbus polling register? I am use to setting a slave address, that a start register lets say "7001" and a ending register lets say "7050". I have looked thru the forum and probably overlooked this. Any input would be helpful on this. Thanks in advance.
Since the Micrologix is the Master, you don't have to pre-assign its data files to Modbus locations. This is something you need to do if you set it up as a slave.
Now that the channel is set up as Modbus master, when you create a MSG instruction and pick that channel, the Modbus command codes will appear as options for message type.
When you enter the MB Data Address, you leave off the prefix. so to read holding registers from 7001 through 7050 you would choose a data file location (probably an N file), enter a size in elements of 50, and enter the Modbus Data Address in the Target device of 7001.
This assumes those addresses are in decimal. Often manuals are written using hexadecimal format.
Anyway once you enter the 7001, there is a read only field at the bottom of the instruction that will display it with the function code "407001"
Be aware that the Micrologix uses "1 based" addressing and some vendors use zero based addressing. Like I found out yesterday with Yaskawa drives, I needed to read starting at address 33 in order to get values from what the Yaskawa book says start at register 32.
There are 3 or 4 things that you have to figure out with any new Modbus RS-485 setup. 1. wiring. 2. addressing. 3. byte order. 4. word order (for reals and long integers).
