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How to assemble the correct RS485 converter circuit?
I assembled a UART - RS485 converter circuit for interrogating the Mercury 206 counter with a Linkit smart 7688 microcomputer on OpenWRT. But I can't connect with him. As a result, I decided to check myself, I bought USB - RS232 and RS232 - RS485 adapters in the store, I bought it that way, because just USB - RS485 was not available. Collected according to this scheme:
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the difference between rs485 and rs232 is that the transmission in 232 goes through separate wires and there is the possibility of duplex (ie, simultaneous transmission and reception).
in 485 transmission goes under the same communication line (twisted pair) and without duplex. Accordingly, it is necessary to switch the driver 485 from the receive mode to the transmit mode.
according to your scheme, this is RE DE (Receiver output Enable and Driver output Enable), while RE is inverse i.e. turns on "zero". I advise you not to forget to indicate the inversion - then it will be easier.
on a computer serial interface, usually ~RE~ and DE were connected and connected to the RTS output.
on the linkit there is only a uart, without control signals.
option 1: rewrite the port driver and output the transmit/receive switch signal to some gpio thread.
option 2: install a chip with automatic transmission direction selection as a driver. google "485 autodirection" mikruhu, there were several of them.
option 3: make autodirection yourself for this, you invert the TxD transmission signal from the uart and connect it to ~ RE ~ and DE.
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