What is the TNOW pin on USB/UART bridge ICs?

Some manufacturers have a pin named TNOW on their USB/UART bridge ICs.

TNOW means Transmit now and it used as driver-enable pin for RS-485. When the TNOW pin is active, the driver is enabled and can transmit data.

Here’s the typical configuration of the TNOW pin (check your specific IC’s datasheet for confirmation):

• When the USB-UART chip is transmitting data on the TXD pin, the TNOW pin is high (e.g. 3.3V) (ie high during the transmission of bytes)
• When the USB-UART chip is not transmitting data on the TXD pin, the TNOW pin is low (GND)

Hence, you can connect the TNOW pin to the DE and ~RE pins of an RS-485 transceiver (just connect all three pins together), to automatically enable the driver when transmitting data

Example IC with TNOW pin: WinChip (WCH) CH9114F