Transferring Data from A/B PLC via MODBUS RTU


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i am trying to the a scaled analog data from Allen Bradley SLC 5/05 PLC to a LT100 (Tbox) RTU.

The MODBUS address of the TAG created in RTU is 20480. I am using RS485 protocol to communicate. P2 LED on MCM module is green. The PLC is in RUN mode, but still i am not able to transfer any data.

Guys any help?
Analog addresses using Modbus RTU are typically 40xxx not 20xxx

Did you set the MCM module up? If so did you use Prosoft's canned programming or did you freelance it?

Is the MCM module is set up correctly in the PLC? Ie for RS485 (2 wire or 4 wire?) or RS232, Master or slave mode? Slave ID? Parity, etc? Are the registers that you want to see mapped to registers that the MCM card is set up to use?

When you use an MCM module in a SLC you have to map the IO address that you want to read with modbus to a register that is used by the MCM to transfer data. Typically that is an Nxx:xx register and typically Prosoft uses N23:xx for reads (PLC ->MCM) and N25:xx for writes (MCM-->PLC) where, in the case of reads, xx is accessed by requesting date from Modbus RTU address 40xxx using function code 3. Mapping would be in the form of a CPY (copy) or MOV (move) block in ladder where I:x is copied or moved into N23:xx

You didn't supply enough information...

I would have used a port server and whatever comm mode device you needed (cell modem, ethernet radio, etc) to read the SLC addresses directly with MODBUS TCP. But everyone has their way of doing things...
The Prosoft module uses M0/M1 files to hold tables of data, and you need to transfer data to the M0 file from the PLC data for configuration and data. you can also transfer data from the M1 table to the PLC for input/status data.

Prosoft has example applications available on their website to show the configuration.

1. Make sure you have the Prosoft ladder routines in your PLC program

2. Check the port configuration for the port you are using is mapped to the correct M0 location, and matches your RTU settings

3. Check the port jumper on the Prosoft module is set to RS-485

4. Check the location / offset of your data into your register table.

A Modbus simulator can be used to troubleshoot/monitor from either end to narrow down issues as well.