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| 11/19/2008 11:12 AM | |
Joined: 9/27/2006 Last visit: 11/14/2024 Posts: 12294 Rating:
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Hello deka; That is because Modbus/TCP specification is defined as a client/server protocol... http://iatips.com/docs/nov2003_modbus_driver.pdf To make things easy, a Modbus/TCP client acts somewhat like a Modbus RTU master, and servers act somewhat like RTU slaves (but not quite). RTU specs are based on RS-485 standards, where the token-passing technique allows only one of the 32 users (max) on the line to force all the others to come to high-impedance on their Tx line, so only one can actually transmit anything at any one time. This is basically an electronic-level control o0f the line drivers of the units. Now, for Modbus/TCP we are talking Ethernet, TCP/IP, CSMA/CD, and all that... So the context is different, you cannot "force" Ethernet nodes to be silent, that would go against Ethernet basic specs... So you design "servers" who's only right is to present a "menu" of data to the "client", who can request specificbits of the menu when he needs it.Imagine a restaurant situation: the client comes in, looks at the menu presented to him by the waiter (the server), makes a selection, and tells the server to bring him his selection, everytime he needs it... Does this help? Daniel Chartier |
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| 11/20/2008 11:21 AM | |
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Posts: 65 Rating:
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Thank you for your answers! |
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