rs485
sorry for my bad english
i have problems with rs modbus , my project is about max 485 i have master and 3 slaves when i put master with one slave it's ok and there isn't any problems but when i put the master with 2 slave or more the rs modbus is disappear ""when i invert A&B in the second slave the led of rx flashed but there is no response from processor""
what is the problem in this??
(first picture for master)
(second one for slave)
the processor is atmega 8
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: -
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)!
Best Answer
Make sure that different slaves have different Modbus addresses.
You have strong biasing resistors (470R), they should not be present on every device. Remove R16, R25, R23 and try. Most of the time the bus will work fine without them.
Remove termination resistors from every but the first and last device on the bus. At low speed (eg. 9600bps) the bus will work even without them.
Try multiple slaves at lower speed (eg. 9600bps). If it works then you have a signal integrity problem.
Test with a known good implementation slave (eg. FreeModbus).
Get a USB<->RS485 converter and test with a known good master implementation on the PC (eg. QModMaster).