is rs485 a communication protocol or modus is a communication protocol?
If we we want to send data via arduino uno to slave using rs485 do we need to use Modbus
?
RS485 is an electrical specification, it specifies what voltage levels bits use. It does not care what data gets sent.
Modbus is a software protocol, it specifies what data is sent, it does not really care how the data is represented electrically.
Modbus is often used with RS485, but can run over other networks. There are many other protocols that run over RS485, and of course you could invent your own, although creating robust, reliable protocols is a bit of an art.
Modbus is the comms protocol. RS485 is the electrical signalling method used to transfer the raw data.
No. Modbus is just one way of doing it. You are free to invent whatever method you like. Have a look at Nick Gammons website as he explains RS485 very well along with some nice demonstration sketches.
No.
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using RS485 you can use simple serial to serial communication without the complexity of modbus
why use rs485? are you transmitting a long distance in an electrically noisy environment?
for short distance (few metres) is TTL should be OK otherwise RS232 may be simpler than RS485
remember using a UNO using SoftwareSerial or AltSoftSerial you are limited to low baudrates, e.g. less than or equal to 38400
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