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It looks like this application wants the flexibility to support either half-duplex (two-wire) or full-duplex (four-wire) signaling - is that correct? If so, this overall topology should work, but I have some minor concerns about how the R outputs are handled. It looks like these are tied to a common net (RS485A_RO), which means that you should be careful not to have the /RE inputs low on both transceivers simultaneously. As long as the RS485A_DE line remains high in full-fuplex mode (so that the U1802 receiver output is disabled) it will be OK.
Since the transceivers each have an /RE control pin, you could consider using these to disable the receive function rather than using the additional tri-state buffer (U1804).
Yes, the application is flexible to support either full-duplex or half-duplex.
From the software driver perspective, the TxEN line will be held high in full-duplex mode and toggle in half-duplex mode.
U1804 is for the DC level shift, the GPIO of MCU only support 3.3 DC level.
OK, that sounds good. By the way, if the MCU only supports 3.3 V then you might want to switch from THVD1550 to THVD1450. THVD1450 is a pin-to-pin equivalent to THVD1550 but can be powered by 3.3 V directly while still retaining compliance to the RS-485 standard and ability to interoperate with other 5-V transceivers.
