Setting up Modbus TCP Network


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I have one application where Micrologix 1400 PLC acts as Modbus TCP Master/Client, and it is connected to 8 Modbus TCP slave/server. When I start communication it gets slower, some slave are not responding.

In our setup we have used DES-1008C 8-Port 10/100 Mbps unmanaged Switch. With same setup if I reduced slaves to 4, then it works fine. So what could be problem?

Here I have one question of switch, which switch should i use, managed or unmanaged?

For future expansion I have to increase slaves up to 24 numbers in one network.
The issue is probably "some slaves are not responding". Although I've seen networks overloaded with Ethernet/IP traffic where the slow Modbus/TCP comm was due to an overload of Ethernet/IP broadcasts. A managed switch helps the Ethernet/IP traffic situation; but will not help a 'slave not responding' situation.

The 'slave not responding' time delay is due to the nature of Modbus is its sequential master-slave polling routine. The master polls and then waits for a response. If there's no response, the master "times out" and goes on to the next poll. So the time-out interval affects the time it takes to get through the list, when there are slaves that do not respond or cannot respond within the time-out interval.