I am working on a project that involves using Modbus TCP to control all the Low Voltage Motors they are around 270 devices. My question is what is the maximum number of devices in a single Modbus TCP Network to avoid problems of bandwith?

 

First I believe you're over the address limit. You may want to break your network up into several smaller networks and concentrate the data (possibly connecting these concentrators with a different type network), and that should help with bandwidth as well. Bandwidth consumption will also depend upon how frequently you need to talk to each device, and to some extent how much data you want to exchange per node.

 

It's unlikely that your 'network' will be the bandwidth limiter. Instead, it will be the master/host/OPC or whatever device you expect to talk concurrently with all 270 nodes.

Who out there on this forum has worked with high-node count on a PC? Anyone running OPC with 300 concurrent Ethernet connections?

 

I don't see a theoretical Modbus/TCP address limit problem in your application. If your default IP.v4 subnet mask 255.225.255.0, then it's 254. But you could extend the mask (255.255.0.0) so its much higher. Modbus/TCP is a light-weight protocol, so even if you botch the implementation, bandwidth is not going to be the show-stopper.

The practical problem you will have is in furnishing a couple hundred network ports. I suggest you get some IT help with that.

 

Well in my architecture I will have 4 MODBUS TCP Interfaces Configured as MASTERS. Each can handle up to 70 SLAVES. The peak throughput of each interface is about 120 request/second. I am planning on a Type A network and will segment it in at least 2 networks of 145 devices each. The SLAVES are my devices at the LVMCC. Each has 32 parameters. 3 COILS for control (Start, Stop, Out-of-Service) and 29 REGISTERS for monitoring temperature, current, speed, etc.

As for now all these devices are TCP but they could end up being RS-485 connected to several Gateways (customer is still evaluating this since it would be cheaper). Which then will give me a whole different problem for response times and my topology will change again. What is the maximum recommended number of Serial devices connected to a Gateway with this type of application?

 

We use 94 PLC's TCP/IP Modbus. These PLC's are all masters with make communication with other PLC's.

12 Modbus gateways with between 30 and 60 RS485 units.
2 SCADA Servers with backups, and parallel Kepware TCP/IP Modbus to OPC.
PLC F.O network switches 10 M
PC switches 1 G

If base on this you see relevant info let me know what I can share.

All the problems that we had where connected to use of sockets. There should be no problem in speed. There could be a problem in real-time with the 485 info but this is application depended.

 

> Who out there on this forum has worked with high-node count on a PC? Anyone
> running OPC with 300 concurrent Ethernet connections?

I have used SCADA Modbus for this number of connections and over 2000 devices ...
http://www.matrikonopc.com/opc-drivers/302/base-driver-details.aspx

Cheers,
Wassim Daoud

 

So Waasim, you are saying a common office-grade Windows PC can maintain 2000 concurrent TCP sockets? I find that a bit hard to believe, although I suppose virtual memory these days can do wonders

- Lynn

 

Well, some changes may need to be made to registry of Windows XP. Later OS will work fine. Linux needs no intro.

The torrent protocol and other p2p protocols can open an amazing number of connections.

Regards,

Tomy Zacharia

 

I also doubt an office-grade PC with Linux can handle 2000 active TCP connections. Even 200 will bring it to it's knees.

I can buy a nice desktop PC for $500 - or a 'server' for $2000. There must be some difference - probably the $2000 server can handle 4 or 10 times the 'server' traffic than 4 desktops at a fraction of the electricity.

Even look up "Ethernet Adapters" online and you'll find some of the most popular ones sell for from $750 to $2000. I mean $2000 for a 4-port PCI-Ex Ethernet card? It apparently helps your $2000 server handle even more traffic for less overall cost-of-ownership.

But in the context of this Modbus thread - "How many Modbus nodes can be on my network?" It is still going to be the Host/Master specification which limits the number. It is not the "Ethernet" or whether you have 10Mb or 100Mb or even 1Gb Ethernet through out.

The large SCADA I've worked with tend to have many blind 'front-end comm' PC which gather the field data each from a sub-set of slaves, then stuff it into a database. The "SCADA" and "Multiple Users" then become a common IT/MIS equation of server/transaction performance.