Hello

For the past few days i been going through the internet for RS485 and its operation with 8051 microcontroller and this R&D gave me some knowledge.


First my R&D shows to me that RS485 defines only the physical layer of signal transmissions. All other things like communication protocols, error handling and others are not defined and they are left to be handled in software.

Second i found that most of the examples use the RS232 for communication purposes... that is RS232 transmits data through a RS485 signaling and on the receiving end this is received by RS485 and send to RS232 port. Have a look on this diagram


http://www.angelfire.com/ak/shahzad73/RS485.pdf


That means that I just insert the RS485 chip in between and assume that I am sending data just like RS232 so use the same software for RS232 for sending and receiving.



So here are my confusions and clarifications to confirm

- First that means that what ever is send will be received by all nodes in the line. I can assume that this is RS232 communication and I am sending and receiving bytes through RS232 software.

- I have to write my own protocols so that a message can be transmit with a address to whom it should go and all nodes will receive that message and decide whether this is for them or not.

- Most important which I can not understand in the PDF circuit that how the collision is detected. I mean at one time only one node should be transmitting. So how the node will decide whether it can transmit or not.


please give comments about any of above three points.


Thanks
Shahzad