Modbus TCP

In a serial RS485 Modbus RTU network there can only be a single master, although that master can communicate with multiple slaves. Does that same restriction apply to Modbus TCP or is it possible to have multiple Modbus TCP masters communicating with the same Modbus TCP slave?

I believe that it depends on the Master and how they are designed and how you use them. In the GEIP world you can have at least 2 (maybe more), of course they need their own IP addresses. In fact some of the GE MBTCP/IP master devices will respond to slave requests from another master on the network. Just be sure that you do not over burden any slaves ability to respond by requesting too much, too often from it.
You know what "they" say about "Too many chiefs and not enough..."

Hello,

For TCP it is socket based, a point to point connection. The slave will determine if it allows more than one socket. Many slaves devices set a maximum number of concurrent connections.

For a master two approaches can be taken. One is that each connection to a slave is via a single socket and the other approach is each socket has multiple connections to slaves.

For each connection has a single socket, the "Unit ID" value of the packet is really not needed because each device has an IP address.

For multiple slaves on one socket, the "Unit ID" is a must so the "slave" can determine which slave is the intended message destination point. This approach is seen on a TCP to serial gateway. For example, one TCP connection to 32 serial devices.

Of course if speed is the issue 32 connections, one for each slave, would be the fastest.

PeakHMI offer both solutions. The number of TCP master to slave connections each with a single socket is only limited by the port number range, 65535. For the single socket for many slaves, each socket can support 255 slaves, the maximum number for the "Unit ID" is 255. 0 is for broadcast.

Hope that helps.

My experience concurs with Russ's explanation.

Modbus RTU's message packet (PDU) does not even have an identifier for a master. There's no provision for replying to a specific master in a reply. The entire Modbus RTU protocol assumes only one master (Modbus Plus addresses these issues with token passing, but Plus is licensed by Schneider). And other than tristating the RS-485 drivers, there's no means of bus control.

However, the overhead in an ethernet packets takes a lot of this into account, making Modbus TCP more versatile.

The Modbus Standard's graphic for a 'typical' Modbus TCP network includes 2 clients (masters) and 2 servers (slaves).


Some devices (single loop PID controllers) will choke if queried too quickly in succession by one master after another, somewhat like a denial of service attack on an internet server.