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Modbus Master Communication between 4 slaves. Modbus Master is communicating with single slave (means RXD, TXD is blinking with both systems, but it can't read the values from Slave). Could anyone help me with this to shotout?

Modbus requires that all of the units, i.e. the master and slaves, have their confiqurations the same: baud rate, parity, etc. Also be sure your addressing is correct. If you are trying to read or write to invalid registers you will not get a response.

Ajith,

I have found when trying to establish serial communications for the first time it's a good idea to slow the baud rate right down, e.g. 1200 or even 300. That way it's more forgiving if there's a timing issue. Then I try different combinations of Stop bits and Parity until it starts to work. Once you have them talking the same language then increase the baud rate.

Another trick I use is a laptop with a terminal emulation program running connected in parallel to TXD and Gnd to display first of all the messages coming from the master then RXD and Gnd to see the response. With this setup it's easy to spot if the messages are good or garbage.

If the connection is 485 the laptop's RS-232 is able to see the signal on a 485 negative wire from memory (I might have the polarity wrong but it soon becomes apparent).

Hope this is useful,
Roy