Control.com - Nerds in Control

How to connect 400 Modbus devices into Modbus network?

As far as I know Modbus protocol only supports 255 devices, and if using RS-485 will only support 31 devices + 1 Modbus Master.

Any comment is highly appreciated.

That's a lot of Modbus. It's going to be slow unless you split it into multiple nodes. Can you use Modbus+ with BM-85s to split it into small nodes?

Roy

Hi there,

I would recommend the use of a gateway. The gateway acts as a Modbus slave towards your master and acts as a master for the slaves that you need to include in your Modbus network. So you can connect 31 slaves to each gateway. For more information go to:
http://www.anybus.com/support/support.asp?PID=317&ProductType= Anybus%20Communicator

or email me at: support@hms.se

Pat

Hi Pat,

Thanks. I'd like to check this Web site and learn more about this idea.

2 e-net ports on 2 subnets?

It is not possible if all the Modbus Devices are on the same communication network because the Modbus address field is only 8 bits wide, thus you cannot uniquely address 400 unique locations.

If you can divide the 400 devices into groups, then you can connect them all to a single master.

The product to use goes by several names such as Data Concentrator, IFE (Intelligent Front End) or Polling Master. There are other names as well, Google any of these terms and you will find more names.

The concept is that all these devices will collect data from one or more independent Modbus networks, each network having several Modbus Slaves and store the data collected into a single "database". That database can then be accessed by another Modbus Master using a single Modbus Address, thus "concentrating" the data from many Modbus Slaves into a single Modbus address.

If you can split up your devices onto several networks, then the problem evolves to being how much data you need to collect (thus, how many "data concentrators" do you need because they will have a finite database size), what kind of response time is required (it takes time to update the database by reading all the Modbus Slaves) and very significantly, if you are going to do controls, that is send a command to change an output from the single Modbus Master to one of the 400 Modbus Slaves, how long does it take for the control to get to the proper slave.

We have products that can function as a data concentrator that you may wish to review. See this link http://www.miille.com/PCds.htm for data sheets on our Omnii-Comm products.

If you have other questions, please contact me directly using the links at the bottom of the referenced page.

Jerry Miille

I concur that segmenting the netorks is the way to go. This can be done with multiple Modbus/TCP to Modbus/RTU gateways. There are many such devices available. Ours can be found here:

http://www.eautomationpro.com/us/product/modl_1-23HZ6X.aspx

Mike Berryman
Advantech Corp