PLC as a Data Concentrator on RS485 Protocol


Thread Starter


I want to use a PLC as a data concentrator unit for my field devices which are communication on RS485 protocol.

I want my PLC to collect data from the field devices before feeding directly to the RTU. As i am planning to add more IEDs in the field, in that case the RTU is breaking connection while communicating with all the field devices for collecting data which are connected in daisy chain.

For that reason i want my PLC to work as "Modbus Master" for the field devices or "Modbus Slaves" for collecting data and then feed these data to the RTU.

Note: I am using a "Comfile Technology" make PLC:MSB632RA-DC which is a Cubloc Studio based embedded PLC, i.e, we can program the PLC from both Ladder logic and by using simple BASIC CODE.

If anyone is having experience working with this PLC or with any other PLC for the above mentioned application, you can reply here on the forum or you can directly reach me at :[email protected].
You certainly have your work cut out for you.

According to the MSB632RA-DC's user manual,
the single RS-232 port is a Modbus Slave port, with a wiring distance limitation of 2m, so the device will have to be in close proximity to the RTU to connect to the RS-232 port.

There is no mention of a resident Modbus master protocol, so you'll have to write the code for the Modbus master on the RS-485 port.

If the box proves too limited for your purposes, you might consider the Red Lion Data Station Plus (DS+), a DIN rail mounted controller. I have been told that as a Modbus Master the DS+ can read Modbus slave devices, place the data in registers and the be a Modbus slave/server which can then be read from a Modbus Master/Client, using either a serial or an Ethernet port.
Hello Gautam,

Though I don't have experience with MSB632RA-DC, I can expect that it will be the same like other PLCs. From my quick read to the manual that available on their website:

I found that it only support Modbus RTU protocol as a slave over serial over RS-232 physical layer, so you would not be able to use it as a Modbus Master.

But it will be possible to act as Modbus Slave to your RTU as long as your RTU support Modbus RTU Protocol as a Master over RS-232, assuming that all of field devices connected to it over analog input, so your RTU just simply gather data trough Modbus.

The Modbus mapping on the MSB632RA-DC, as I found on the website will be as below:

Curt Wuollet

This sort of thing is straightforward IFF you have the ports to do it. For a recent project I have an inexpensive AD Click that is a slave on two networks and a master on one. It lets me (easily) have two HMIs, which need to be masters, on one PLC. One HMI is a RS485 master and the other is a Modbus/TCP master. And the PLC is the master on a conveyor network. This kind of connectivity is getting more important and doing it inexpensively is impossible with many PLC brands. The PLC cost less than just the adapter needed for any Modbus 485 with say, an AB Micrologix.

First of all you must design your network, whether serial or ethernet. In your case, I assume you will use RS-485 2W networks from PLC to your field devices. If you plan so many devices, you will need more than 1 port of RS-485 on master side to gain optimized scan time. Altough modbus over RS-485 can support slaves up to 247, it will be trade off between total slave in a network and master data scan time.

I have ever do similar job as you. I used Modicon M580 PLC as master, has 8 serial port, each connected to 3-4 slaves. The M580 PLC , on the Ethernet side, act as a Modbus Server and the RTU will read all set of data (processed or raw data) from M580.
How about trading your many hours of labor for the cost of an Allen Bradley CompactLogix and a ProSoft MVI69-MNET module installed? It becomes a walk in the park if you use the right hardware.