RS485 is bidirectional does it mean it is fullduplex?

I was told by my professor that specification of RS485 is a half duplex.
RS485 four wire mode has got four pin Rx+, Rx-, Tx+, Tx-. Where the master
device Tx+ and Tx- will be connected to the slaves Rx+ and Rx-
respectively. Simlarly Master's Rx+ & Rx- will be connected to the slave's
Tx+, Tx- . Other than the master all the device will be in receive mode, so
if only there is the transmission RTS is pulled 1 and they(slaves) can

The master is always in receive mode and can also transmitt. In this case
the master is in the fullduplex mode, it is violating the RS485
specification! So could anyone guide me to the right direction regarding
what I have understood.

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