I want ton know how multidrop differ from daisy chain topology? I am using RS 485 standard and modbus protocol for communication. So which topology is best for my requirement ?
Electronic – Difference between Multidrop and daisy chain topology
modbusrs485
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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 & 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.
Yes, obviously you need an UART. Your driver looks also okay. One UART is enough. Do not attempt to build your project when you are not familiar with terms like: Bus system, RS485, Physical Layer, UART, RS232, half duplex, Master-Slave topology, bus-arbitration. With that you will understand what a bus system is and how multiple devices on a single line work. A good starting point are application notes from some rs485 vendors and tutorials, for example this one.
Personally, I used the ST3485, which has an application note explaining some basic stuff here but you can find the same information from all other vendors also.
For your Firmware and Protocool decryption there are many librarys out there. First of all you have to know if you want to build a Master or a Slave device. For all of the above: Google is your friend. But for google to find the right stuff, you need to know the terminology (what to search)!
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Best Answer
They are the same thing in terms of RS-485. Electrically identical. Multi-drop is the more accepted term.
To put a finer point on it, daisy-chaining is a connection technique whereby one RS-485 adapter is wired directly to the next one, and so on.
The alternative connection technique (which would not be called "daisy chaining" is to have a common bus architecture, and just have each adapter connect to the bus).