Anyone interested in Modbus can find specifications at
http://www.modicon.com/techpubs/TechPubNew/index.html.

The serial protocols are described in
http://www.modicon.com/techpubs/TechPubNew/PI_MBUS_300.pdf

The Modbus/TCP protocol is described in
http://www.modicon.com./openmbus/

BTW: there are two serial protocols called ASCII and RTU. They differ only at what you might call the data link layer. The higher layers are the same. (Though you won't find any description of layers in the protocol specification, it can be implemented as a layered protocol.)

ASCII is easier to implement, but RTU is about twice as efficient. As a result most low bandwidth applications use the RTU protocol. I'd recommend implementing both, but if you have to choose, pick the RTU protocol.

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Hello Sir, My name is Arun Mallah and I am interested in getting the Specfications about the Modbus Protocol. I have visited your Sites also but was not able to get the core information which I am Looking For.I will be very thankful to you if U tell me the core information about it's developement. Please Reply as soon as possible at my above e-mail. Thanks -Arun Mallah