which device will be slave/client and which one will be master/server

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hi
I Have one basic doubt,I was reffering the specification of MODBUS TCP,
I   got confused.
According to my understanding, the controlling device i.e the RTU(Real
Time Unit) will be the slave and the one who request(poll) for the data
is the master.
But in case of client/server concept the one who serves the
request(RTU) will be the server and one poll for the data is the
client.

Please clarify my doubt
Thanks in advance
Prasi


Re: which device will be slave/client and which one will be master/server
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That sounds correct.

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The master/slave allocation is a low-level communication synchronization
concept.  It says nothing about which end requests data.  You may have
one master and several slave units.

Either master or slave may make a request of the other unit, depending
on how the system is designed.  The client/server model is better suited
for independently running clients.  A realtime system is usually more
coordinated and doesn't fit the client/server model as well, especially
with cental control.

--
Thad


Re: which device will be slave/client and which one will be master/server
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That's not true in older Modbus (non TCP) systems.
Traditionally, Modbus is a single master bus.  Only the master
my make a request.

Is Modbus TCP multi-master, or has the master-slave thing been
thrown out completely, and it's all peer-to-peer where any device
can send a request to any other device?  In that case, all
devices are both clients and servers in TCP terminology.

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Re: which device will be slave/client and which one will be master/server

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RTU stands for "remote terminal unit".  It's generally a slave
device that provides I/O points for use by the master.

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Correct.


Exactly.

The RTU (the remote I/O device) is the slave.  In TCP terms,
it's probably the server. The control system that reads/writes
data to/from the RTU is the master (the client in TCP terms).

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