UART signals connected to RJ45 connector?
UART TX/RX signals to a RJ45 connector  [...] Does anyone know such application?


Yes, it is common with various IT devices e.g. terminal servers, some rack-mount and blade servers etc. The specific connections (wiring) between the serial signals (often RS-232 rather than UART level), and the RJ45 pins, can vary.


  What's the purpose of such design?


RJ45 connectors and suitable cabling are common, and take less space on a PCB and back panel than other "standard" serial connectors like DE-9 (some servers which have very limited space on their back panel and PCBs).

Of course using such connectors for a serial interface has the disadvantage that they can be mistaken for an Ethernet connection.



Your newly added block diagram does not show the signal labelling expected for a typical RS-232 serial connection from a "UART to RS232" block. Your signals labelled RX_DP and RX_DN suggest they are the positive and negative differential pair for the RX signal. RS-232 is not differential. Therefore the various parts of the block diagram do not match (i.e. non-differential RS-232 with (implied) differential signal pin labels on the RJ45 connector). If understanding that detail is important to you, then more information is needed.