When the interface MCU receives a Data Terminal Ready (DTR) signal from a terminal, it performs automatic Hardware Flow Control (HWFC) detection.
HWFC detection
Automatic HWFC detection is done by driving Clear to Send (CTS) from the interface MCU and evaluating the state of Request to Send (RTS) when the first data is sent or received. If the state of RTS is high, it is assumed HWFC is not in use. If HWFC is not detected, pins (CTS) and (RTS) are free for the nRF application to use.
After a power-on reset of the interface MCU, all UART lines are tri-stated when no terminal is connected to the virtual serial port. If HWFC has been used and detected, (CTS) is driven by the interface MCU until a power-on reset has been performed or until a new DTR signal is received and the detection is redone.