How many wires needed in RS485 half duplex communications?


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How many wires do we need in RS485 half duplex communications?

1 cable for rx+
1 cable for tx+

Is it necessary to have a ground?
It is said that at least 3 wires are necessary but I can communicate with a pm800 with only 2 wires (rx+, tx+).

With 2 wire communications:
During transmit, does rx+ act as tx- and during receive does tx+ act as rx-?

And what if I use shield for ground and earth/ground both?
The +, - shield configuration of 485 is referred to as 2-wire RS485 typically. My understanding of the role of a shield on a 485 network is to act as a baseline for the signal generating electronics, but I don't have a whole lot of knowledge on the electronics side of it. I would expect that there would be cases where 485 networks could function without a shield just as I have used them plenty of times without any appropriate terminations.

Also, in 2 wire mode I believe data goes over the pair of wires in one direction at a time between devices, therefore it is half duplex, like speaking to someone normally on a telephone where you take turns.