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