Arduino PC communication(SEL SDATA pins)

Hello, I'm making a proyect, and I need tu communicate my Arduino DUE with the PC. But there is a problem. It needs to communicate with a PC through a private stuff(device) that I cannot look how works.

I know I am not giving much information, but I haven't got much more.

The point is that the stuff which receives the data I send with the DUE has 3 pins, and its names are SEL, SDATA and GND.

So, if I am not wrong, I2C uses SCL(clock), but not SEL. In this case, the SEL pin gives the information to tell the slave(DUE) if it is reading or writing(R/W).

I tried to send data with the Arduino using Serial Tx(Tx--SDATA) pin, with a baudrate of 2400, and it works, the PC software shows the data perfectly :fearful: . So, I guess the protocol used is UART.
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But when I send data from the PC to the DUE into Rx(SDATA--Rx) Arduino doesn't recive nothing.

Does anyone know what type of communication uses SDATA and SEL(R/W) pins?