atmega32 communicating using rs-485 modbus protocol
The WDM303FDWA1 uses RS485 as the line interface connection and this means you need to interface the Atmega to an RS485 chip such as a MAX485. Here is a link to Maxim's offerings on that device and here's the chip: -




RO is received data out from the 485 and connects to your Atmega
RE (bar) is read enable driven from your Atmega - it defines data
direction along with DE
DE - tie this to RE (above) and you have one pin that controls
direction of data on the 485 line.
DI - this connects to the data output line designated for your
Atmega.
A &amp; B are the 485 line connections


If you look on page 8 of this document it details all the codes you need to send to talk to the device. It appears that you must send the "password" first followed by a function code, then the specific address of the data you want to read. The pdf file has that data.

As for the format of what comes back I can't help you with that - the document doesn't go into too much detail on this but, it does appear to be between 1 word and 3 words.

Neither can I help you with samples of modbus codes for use in the Atmega.