Modbus TCP bBaud Rate

C

Thread Starter

chodj99

hello

i have a question about Modbus. i'm going to use Modbus/tcp for my 10 slave PLC to connect to master PLC.

if some slave PLCs are broken and Modbus/tcp is disconnected, does it
affect other slave PLCss communication speed with master PLC?

Hope you guys have a nice day and look forward to have a kind answer. thank you.

my email is [email protected]
 
I

InterwebStranger

Modbus TCP is not usually daisy chain network. The Modbus messages are distributed using TCP/IP over the physical Ethernet network (with varying topologies such as star or ring). The Master (client) establishes connection with the slave (server) by following the TCP 3-way handshake (Syn , Syn+ack, Ack), and the connection usually stays open until the client requests to close, or a time-out occurs. Some implementations open an active session for every Modbus function call, and subsequently close it when the Master receives the response.

The throughput ("communication speed") will be determined by the poll rate you set for each function call to the slaves. There is plenty of bandwidth on 1Mb to 10Gb Ethernet.

Anyway, the point is that if a slave stops responding, the Master will timeout and will try to request a connection to the slave. This won't affect other slaves.

In another scenario, a slave might have a maximum number of TCP sessions that it can service at one time. If a master continually opens new connections (and doesn't close them), the slave may need to be reset (power cycled).

Does any of that make sense, or is this just late night rambling?
 
Sorry for late reply.

Thank you so much.
It's really helpful for me.

Thank you! wish you have a nice day!
 
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